For awhile I was managing to play multiple MMOs of interest by picking a single character in each and just working on them, not messing with alts.
The problem with this is, particularly in a game you don't know well, at some point with an early level character you can get stuck. It might be right around level 20. Suddenly, you can't seem to move forward.
Sticking to the one character per game rule, you often end up not playing that game. Maybe you pick it up later, maybe you never return.
Many games seem to limit you to just a few characters, sometimes only two. In order to try out a different class which might make all the difference to your playing enjoyment, you have to kill someone off or buy a new character. Why are you going to purchase a character you may not like any better? Maybe it isn't the class, it's that the game isn't for you.
Enter glorious World of Warcraft with eleven different classes, each with three different role specializations. Fifty five character slots to play with.
People wonder why they keep being drawn back to play this game, and having that kind of variation and ability to try different roles and servers, and do every single thing the game offers (and it offers a lot) for the price of admission is a huge draw.
I have two main servers I play on, that's twenty two characters I can return to and work on anytime. With more opportunities for further adventures anytime I'd like. I have a few classes who are played little: Shaman, Monk, Warrior. They're always there, though, waiting their chance. I don't have to delete them to try something else.