Two Questions:
1) Should the reason for their absence have any effect on whether or not they get experience?
There's a difference between mother in the hospital, double-book with a pub crawl, and just forgot as excuses.
If their mother's in the hospital, I'll express my sympathy, and otherwise proceed as normal. If they double-book with a pub crawl, I'll get a bit miffed they didn't invite the rest of us along, and then proceed as normal. If they just forgot, everyone in the group will mock them for the duration of the campaign and find ways to annoy them with "helpful" reminders leading up to every session... and we'll just proceed as normal.
If their behaviour becomes an issue, then we'll discuss their behaviour. Their character's rate of levelling will not be affected, unless their behaviour has become enough of an issue that we ask the player to leave the campaign, in which case their character probably wanders off into the murky woods never to return (unless someone calls dibs).
2) Levelling everyone as a group removes the ability to reward creative thinking, role-playing, good behaviour, and the like with experience. Should alternative rewards be in the game? Either as part of the core rules or as a module.
Yes, it removes the ability to reward those things with
experience points, unless you reward the whole group simultaneously (which remains an option). It certainly doesn't do anything to stop us from rewarding players and PCs in other ways, many of which are more meaningful than a chunk of abstract points that, in the grand scheme of things may not even lead to a single session advantage in levelling. In game,
in character, rewards are (to me) generally preferable to metagame rewards. Would it be a good idea to discuss some of those ways in the DMG? Absolutely. That seems to me to be one of the purposes of the book.
Really? I think you're rather underselling the basic notion of a game (in which achievements defined by the rules are attributed meaning by participants and observers). For example, I think many people feel rather strongly that the San Francisco Giants recently achieved something greater than "fun" simply by playing a game (while the same "fun" is not much of a consolation prize for the other teams). The sense of achievement from performing well in a game, arbitrarily defined as its rules might be, can be a major driving force in people's lives.
I think the number of ways in which my D&D campaign is different from professional baseball is quite a large number. For one thing, none of us being payed even in peanuts to show up at the table and play. None of us in under contract. We are none of us supporting our families, lavish bachelor lifestyles, or future retirements off of our D&D earnings.
We have, usually, no non-participating spectators. Nobody has bought tickets to watch us play.
There's no D&D Hall of Fame in Cooperstown waiting to enshrine our character sheets, record our XP totals and performance metrics as a shining example to fans and future players.
Few, if any, of the players in an MLB game (nevermind the World Series), are there simply because they get enjoyment out of playing. Most of them, if instead of being payed tens of millions of dollars, they were offered access to a communal bag of chips, bowl of dip, and a box of TimBits (all paid for by the players themselves) as their only reward beyond the fun they can find in the game, would probably find something else to do with their time. It's their job. I would hope they find some enjoyment in their chosen profession, but hey, if they don't, they're rather handsomely rewarded for their suffering. If, in addition to the snacks (but still instead of money) we also offered arbitrary points which can not be converted into goods or services, I don't think they'd find that additional temptation sufficient to continue playing a game that (far more often than D&D, at least the way I play it) can lead to the need for reconstructive surgery on major joints.
My players, on the other hand, would probably still show up even if we didn't have snacks (well, maybe not... they incorporated their desire for snacks into the adventuring party's in-game name), and certainly wouldn't fret too much over the absence of XP rewards.
There's also the creative aspect of the game, which can constitute artistic achievement. Not that many people will ever see it, but there are many great artists toiling away in obscurity, some under the auspices of playing an rpg.
Neither of these precludes the recreational aspect of the game, but I'd say it's entirely possible that D&D is more than just fun.
Certainly, I can see the motivation to want to recognize player achievement by "keeping score" using XP, even though I don't do it.
I certainly won't deny that there's a creative aspect to the game, but I consider that aspect a subset of "fun". I suppose there's some tortured artist types out there who manifest their creativity through D&D play whilst
not enjoying themselves, but I'm not sure I'd want to play with them. Watch them, maybe, but I'll stick to playing with folks who enjoy playing.
I just don't see the point of keeping score. There's no leader-board. The max score is achievable by everyone, requires no particularly great skill, and I don't know of anyone who would be particularly impressed to find out that my Half-Elf Rogue managed to hit max level three sessions earlier than anyone else in the party.
In systems where the metagame points can be traded in for in-game benefits (beyond just levelling up), then sure, there's some reward value to the points. I don't particularly care for that design approach, though.
All that said, yeah, if folks get a kick out of keeping score, have at it. It's not going to harm me in any way, as far as I can tell.