I can usually maintain 2 alts' worth of activity without difficulty. I've got 4 on TLF right now -- this happens when I get really invested in a game and can't make the character concepts in my head go away. Our activity requirements for non-responsibility-having PCs are pretty light, so I manage it without much difficulty. In terms of creating meaningful relationships with that many characters ... well, I conveniently RP each alt more when the players who have meaningful relationships with those characters are available!
Now when I get the character creation urge, I write plot and put NPCs in it. (This backfires -- my fourth alt on TLF was a NPC from a plot I ran that I loved so much I couldn't get over my desire to keep her.)
I see no reason whatsoever to limit people who have the time and attention to spend on a character and aren't creating problems. And if they are creating problems, I don't see how more characters or less characters is going to make a difference. Assholes gonna asshole. If a player's a pain in the ass, how are they less of a pain in the ass if they only have one character? What I do find annoying is when people app a million characters and then idle on all of them, which is why reasonable alt restrictions I'm in favor of, because this slows up the bus and interferes with other people's play. But if that's not happening who cares if someone is playing 2 or 3 instead of 1?
If they're meeting activity requirements and enjoying the story, great. It means there's more variety of characters doing stuff. To be frank, I probably wouldn't even app on a game that required one character only per player, not because I always have the urge to alt immediately (I don't), but because it reflects an attitude that isn't compatible with my playstyle.
But I've never played WoD in my life and as much as I've read about it on these threads I still don't have that clear idea of what all y'all get up to.