The section on awarding XP in the 4E DMG is short at just three pages yet there's a paragraph or two there that has drawn a reaction from some GMs, especially the more traditional ones. Under "Experience at the Table", there is section labeled "Absent Players" and one of the advice it gives is to reward the same amount of XP to players who missed the game compared to those who were present. Of course whether you subscribe to that belief or not, a pet peeve of mine are people who whine about it because 1) that particular section is simply a recommendation and if you don't like it so much 2) you can always simply reward XP "the default way". What the DMG does provide are options for GMs and whether you accept that method or not probably isn't as important as being consistent (which is also mentioned in the chapter).
First, let's tackle the old method of awarding XP: you show up for the game, beat monsters, steal their loot, and gain XP. There will be variations in this formula such as the GM awarding you for roleplaying, completing quests, etc., but the basic premise stays the same--you show up and participate in the game and that's the only time you get rewarded with XP. This formula is pretty consistent across a lot of RPGs, including video games and MMORPGs. In fact, that's how MMORPGs can become a time sink, when you spend much time leveling up your characters so that they can be "fun to play". The boring part of this experience, especially at low-levels, is usually what's called "grinding".
Now this first method of awarding XP is quite meritocratic. If you don't show up for the game, you don't get XP. Jeff Greiner of The Tome pretty much sums it up as the "carrot and the stick" approach. It's the basic method of enticing players to continue playing the game: the more you play, the better you become. Another variation of this is treasure (the more monsters you kill, the more treasure you get). As a GM myself, I understand why GMs will take this approach to awarding XP. I mean irregardless of how many players show up for my game (assuming the minimum amount of players is attained), I'm still 1) preparing/running a game and 2) spending the same amount of time preparing the adventure.
However, this method isn't without its flaws. What if you do have absentee players? Following this method, characters will end up with uneven levels. Let's say the highest-level character in the party is 3 levels higher than the lowest-level character. That's great news if you're the character with the highest level but what if you're the one with the lowest level? Suddenly, you're less efficient as opposed to the rest of the gaming group and on the side of the GM, it messes up my math when accounting for challenges to throw against the party. Now you might say "but D&D is a cooperative game". Sure, that's true. But let's say the absentee player is playing the role of the Defender in the group. Does it benefit the party that he has less hit points/attack bonus/powers than the rest of the players?
The larger dilemma however is that this solution does not encourage absent players to return to the game. For example, let's say a player has missed ten gaming sessions (irregardless of the reasons for doing so). Do you think playing a game where he is at a probably-ineffectual level will make him want to return? In MMORPGs, a character lagging behind in levels has various options available at his disposal to help him catch up to the level of the other players, everything from the higher-level players giving him loot that they don't need or actually spending the time with him battling foes they easily scoff at. In a tabletop RPG, there is no such option--at least without taking much time away from the rest of the gaming group or deviating from its regular plotline. There is no "grinding" in D&D, unless the GM provides the option to do so (and at this point, if grinding is unfun for your players, why put it there?). Some GMs will usually hold a "make-up session" for lagging players to catch up but in the bigger scheme of things, this expends more time on the GM all for the sake of a meritocratic ideal (and if a player is lagging behind by 3 levels, is it realistic that he'll level up three times in just one session?).
Again, as a GM, I understand the emotion "I worked hard to prepare this game and it's his/her fault for not allotting the time necessary to participate in the game". However, at this point, you have to ask yourself the question: do I really want to game with this person? If yes, then why penalize him/her for wanting to return to the game? If not, then it's probably better to communicate to the other person that you don't want to game with them rather than use XP rewards as an excuse to discourage them from playing unless you're the passive-aggressive type.
Now let's move on to the second method: awarding XP to players equally, irregardless of whether they show up for the game or not. As a GM, this makes awarding XP faster and easier. It might not be meritocratic, but at least when I come to the table, I know the party's actual level and what kind of challenge they're capable of facing. Second, both as a player and as GM, I can focus on the other aspects of the game, whether it's the roleplaying, combat, or whatever else catches your fancy instead of worrying whether I'll get XP for this or that. Now I'm not saying this method is for everyone. If you're gaming with a bunch of acquaintances or strangers, feel free to use the more traditional method of awarding XP. However, these days, I'm gaming with friends, people I've known for several years. The relevant question I'm asking is "irregardless of in-game rewards, do I want to game with these people?" My answer is yes and so I play with them.
A complaint regarding this method is this hypothetical scenario where a player might simply show up for only during the first and final session. What then? Again, let me ask you this question: do you want to play with this person? If the answer is no--and feel free to explain your reasons to this player--then simply don't game with them. You can tell them "I'm sorry but many of the other players have invested in this campaign and I think it's intrusive if you suddenly popped up for the final session." If your answer is yes, then it's in your best interest to give the player a chance to have the most fun, and this typically means starting out on an equal level with the other players. Doing so may not pay-off now, but after that last session, it might spark that player's interest in gaming again and ask if he/she can join in your next campaign where hopefully he/she will have a more consistent attendance.
If you feel you need to use a "carrot and stick" method to entice players to game, then do so. However, I've come to that point in time where I feel me and my fellow gamers are mature enough not to need in-game mechanics to entice them to play. If they want to play in my game, they're free to do so. If they don't want to, for whatever reason, they're free to do so. Sure, there's a possibility that I'll get absentee players, but I want to shift my concerns from rewarding players to making sure that they have the most fun. If other people want to game with me, they'll game with me because they want to, not because they're pressured to level up.