I just read an article someone posted a link to in a G+ community. The blog post was talking about doing away with XP progression.
The person who linked to it found it interesting while disagreeing with it at the same time. I read it and can't say that I agree or disagree with it really.
The blog post author thinks XP is stupid, troublesome and doesn't really reflect credible reasons to affect level progression.
I say that it really depends on the DM/GM and how they run their game.
For example, what's the intended/desired result of playing the game? I wrote a post some time ago called "How do you 'Win' AD&D" or something like that.
For players, it's obvious in how you "lose" the game, your character dies. Game over for that PC.
In that post, I talked about tying game points to how many "kills" a PC makes in a game adventure. It doesn't have to be kills, it could be anything the DM has the PC's doing the most in a given session. Stealing or "recovering" a certain number of things, pieces of gold found and collected by each individual PC, and so on. The DM is limited only by their imagination in how to get points available to PC's so that Players can accumulate and tally points then "win" a game.
Of course, there are other ways for Players to "Win" a game adventure or session. Each PC could be in competition with the others in accomplishing a certain ask or mission. First one to succeed wins. They get to be listed as "Player of the Week" or something on their groups website or whatever. Maybe everyone pitches in five buck at the beginning and winner takes the pot at the end. Who knows really.
What I'm getting to is that depending on what the DM/GM sees as the final objective of the game, how the game is "won" and ends, so to speak, is perhaps the biggest determiner of whether or not XP is needed or desired in their game.
For example, if whoever is the highest level at the end "Wins" then XP for the purpose of level will be very important.
If whoever has the most gold and treasure at the end "Wins", then XP really isn't necessary. A DM can pretty much determine, based on the challenges placed in the game, what levels the PC's need to be at to even have a chance and start them off appropriately as they decide. XP not needed, especially if the game is jutst a one shot.
If the DM decides that he/she really wants to make sure Players have a good grasp of how the game is played and that the Players are plying with skill and creativity before pulling out "the works" as they would like to, then using XP for level progression might make sense to gauge how the Players are doing as they handle their PC's from one adventure to the next.
Perhaps once the Players are playing at a level of play that is satisfactory to the DM, then XP can be cut out and the DM can just throw what they want at the Players and the Players can decide which characters they want to play with at the proclaimed level the adventure runs at and level up to that point as necessary. No good reason not to if the DM and the Players are all good with it.
The only time XP is really truly "necessary" is when the game is being played in a competitive format such as at a convention or something. Players accumulate XP and level a PC as they make their way from one gaming table to the next higher one as they win and ascend till they get to the last, championship table.
Last PC/Player standing wins.
Ultimately, my point is, XP is not stupid or useless or great or awesome or whatever. It simply is what it is as each individual DM's game world calls for it.