Wow! A great discussion. A lot of issues. Here's a set of observations about the posts so far:
1. The RAW
It's pretty clear that the rules as written see experience points as a reward to the player rather than the character. In our culture's stories, people who come back from the dead tend not to come back less powerful. Gandalf, Obi-Wan-Kenobi, Jesus, etc. are more powerful when they return from the dead. And yet, when a character dies, there is an XP loss. This seems to model the game punishing a player for letting the character die.
Furthermore, the things that can be purchased with XP often don't appear to be linked to what one would normally see as arising from experience. People can use XP to buy magic items and other things that add to their CR like Awakened creatures, bound creatures and Allies. Similarly, when leveling XP buys knowledge of hitherto unknown languages, skills, feats and spells. A wizard who never casts a spell and hits things with a quarterstaff learns the same number of spells as one who lives entirely by magic.
Then there are suggested methods of awarding non-CR-based or bonus XP. It is suggested that a GM might reward a player for playing in character or otherwise roleplaying highly effectively. Clearly this doesn't model any game-world event. It's just the character being him/herself.
So, it appears that the PHB sees XP as going to players to spend on their characters not as a character attribute in and of itself. After all, XP can purchase attribute increases (at levels 4, 8, 12, 16, 20).
I'm with Grimstaff that if you give out XP for missed sessions, you really should houserule away XP loss for undead attacks and death and XP gain for roleplaying.
2. Playing With Friends
My last three gaming groups have been happy and good-natured for the most part. We're mostly professionals or graduate students in our early 30s. We typically meet every second week. However, some members of the group are more committed than others. Some will skip a game rather than postpone or reschedule a date or plan a holiday long weekend trip that conflicts with the game. Others cut holidays short and reschedule (or don't go on) dates in order to make games.
Now, I could treat both groups the same and give both the same experiece points. But, I find that things actually go better when those who are not getting fulfilment from other priorities in their life get to feel they're getting more out of the game. And it's not like those with more diverse priorities fall indefinitely behind. In fact, the XP system, over the course of the 20 levels allows players who attend 75-80% of the sessions to stay about two levels behind those who attend every one. And, as others have mentioned, most published modules are designed for such groups.
I don't feel like my game is somehow less friendly because my players aren't all of an identical power level. Weekly poker games do just fine with financial and skill imbalances; I don't see allowing those more into the game to get more out of it as any more or less sociable than giving everyone the same XP.
3. Consequences
I'm really committed to the idea of losing XP for dying. Most other things causing XP disparity I am less committed to. But I like the idea that my choices as a player have real consequences for my character. Unless your world prohibits ressurrection of characters, XP loss is the only way to make the death have any consequences at all. BY the same token, I don't want my character gettign XP when I'm not around to take those risks.
Furthermore, when I run a game as a GM, characters whose players are not there are not really part of the narrative. They're off fighting other creatures off-frame in combat; they don't have anything to say in NPC dialogue. Often they're not there at all but instead shopping or guarding or whatever. As they're not risking death in these situations, even if you buy the idea the characters are getting the experience and not the players, why should the characters get it -- they're not in life-threatening situations.
4. Realistic XP
If you want an experience system that takes a shopt at modeling reality, use Runequest or some other game. In Runequest, your skills only rise when you use them. You can't fireball a land-shark and, as a result, gain an understanding of ring forging, the orcish tongue or how to tumble through crowds. Fireballing a land shark gets you better at fire-balling land sharks.
So, I just don't buy XP is designed to model character experience; it's designed to model player experience.
5. Extended Absences
Sometimes people have to be away for a long time. This is inconvenient in that their character, at some level, drops out of the story. In my view, the best way to respond to this is to reintroduce them with a new character at a level at or close to the party's.
If the extended absence has been due to a loss of interest in the game, it often helps to rejuvenate it by giving the player new opportunities and challenges. If the extended absence has been due to some other, more serious issue, I'd recommend the person generate a new character for much the same reason but if they insisted, I probably would make an exception and top up their old character's XP.