A fan asks that question, which lies at the base of all Diablo III end game debate. Bashiok replies, but not to the OP, presumably since he’s tired of making the same points over and over again. He does quote and reply to another post in the thread:
I’m sure you’ve read this several times, Bashiok, but my stance is that gaining levels are great rewards, and extra levels would make great long-term goals for the end-game. Not to the extreme of 99, mind you. Maybe an extra 5 or so for players that like grinding out that extra bit of advantage.
There’s no such thing as extra levels for people who want to grind them out. Either they’re there and provide a necessary boost in power in which case it’s not a choice to get them or not, or they don’t exist. We’re not going to put in extra levels that provide no bonus simply so people have achievements to work toward. That’s what achievements are for.
I hate to make the comparison, though it’s been pointed out many times before. But D3’s end game is very similar in theory to WoW’s. In both games, characters will reach the max level just clearing the main levels and quests; you don’t have to do extra grinding to level to the max. Thus, the end game in D2 (and WoW) has nothing to do with gaining experience. It’s about finding better items, PvP, and earning achievements. The mindset that wants it to be about exp grinding comes from earlier Diablo games, and it’s one that’s not applicable to D3’s end game. For better or worse.
Bashiok has said that there will be numerous cool things to do, in the end game, other than just running Diablo (or whoever) over and over again. Since the team isn’t ready to reveal any of those yet, they can only hint at achievements. We can only take their word for it, until we’ve got some actual data to discuss. Which might come at Blizzcon, but will most likely not be revealed until much closer to the game’s release.