Unless you clinch a special pre-order deal for the last days of an open beta, playing a beta means playing a character that is bound to get deleted. Some games reset several times during the alpha / beta testing phases, but all games reset at least once before the release. When you create a beta character you know that he is doomed to get wiped. Which doesn't do much to diminish the fun of playing a beta. Because the interest is in the playing, and maybe in having some influence by reporting bugs and making suggestions, and not in building up a character to some level.
Playing on a fresh server, where everybody is level 1, is fun. Whenever World of Warcraft opens up a new server, there are people abandoning their old characters on older servers to start afresh. A new server gives you better opportunities to find low-level groups, and the economy isn't spoiled that much by mudflation yet. But the older a game gets, the fewer new server open up. WoW had new servers for the Burning Crusade, but I don't think they opened up many after that, because the player numbers have stopped growing in the US and Europe. Vanguard, only 7 months after release, is actually eliminating servers, merging 13 servers into 4. Many other games just reached a stable level, with no new servers in sight.
MMORPGs are persistent worlds. With storage being cheap nowadays, old characters aren't deleted. When I logged into EQ2 after nearly 3 years of absence, my characters from 2004 were still there. In World of Warcraft the first expansion, the Burning Crusade, caused a large number of players to resubscribe, confident that their old level 60 characters would be still there, ready to be leveled to 70. And me, and many others who left WoW since, will resubscribe again for the Wrath of the Lich King, and level up to 80 this time.
But persistency has it's price. Developers have to provide for characters that never die, but continue to want better and better stuff. Thus we get mudflation, with every expansion giving us new loot which makes the old loot obsolete. We also get an accumulation of characters at the level cap, while the low- and mid-level zones become increasingly empty. And that messes with the economy. Most lower level characters you see are twinked or are being powerleveled by friends to rush through the game and reach the end game.
In some special cases persistency turns out to be even more counterproductive. Several upcoming games, like Warhammer Online or Pirates of the Burning Sea, have some form of Realm vs. Realm PvP. And if one realm wins this war, then what do you do? You can't just let the servers be persistent, because the winning side controlling most of the territory isn't likely to be overthrown by itself. So these games introduced the concept of PvP resets. The players of the winning side get a medal or something, and then territorial control is reset to the initial situation, to allow the war to recommence, with the former losers getting a chance to win the next one. We'll see if that works (it might be that the same side wins over and over, due to some numerical or game inbalance), but the concept of sacrifying persistency for the greater good of the game is there. So what if we would extend that concept to levels?
Why not have a non-persistent server or even non-persistent MMORPG, which resets every couple of months? Once or twice a year all characters are wiped and everybody has to start over at level 1. You can have rewards for having reached the top in the previous round, like unlocking new character classes (like the Deathknight in WotLK). And you can also do major changes during the reset, like fixing balancing issues, or patching in major modifications to how some parts of the game work. A Tale in the Desert works like that, with every reset improving and enlarging the game.
The advantage of having resets instead of persistency is that you can have an actual end to your game, major events that change the world before the reset. Because persistent is often also stagnant, a reset allows you to break out of the rut. If done well, the players can have a major impact on the virtual world, making their actions really change something. And instead of every expansion adding new content to the end of the game, every reset can add more content to the whole level range. Older players will have new content to explore when leveling up again, while new players can mix with them more easily and play with them at the same level. Veteran players retain the advantage of superior knowledge, but there is no more mudflation, twinking, or powerleveling.
People get attached to their characters, but the price we have to pay for that are static worlds, and an ever-growing divide between new and old players. Having occasional resets allows the game to refresh itself, to rise like a phoenix from it's own ashes towards a new start. Maybe such renewal can bring real longevity to the games we play.