We have a successful guild that has less than one alt per person in the guild on average but we hit the 500 cap constantly. We have between 10 and 40 people on 24/7 with very rare exception and we don't invite "levelers" or pve-ers that don't raid.
The 500 limit is a real pain in the ***, because sometimes the best players will leave the game for a while, but may come back six months later and we cannot leave their toons in the guild because of the stupid limit.
It's annoying. I do think there should be a limit for guilds, but I think 500 accounts should be the limit, not 500 toons. This would still have the same limitations on chat/instances but allow people who want to play some alts but still be in the community of our guild to stay and that way we don't have to delete old IMPORTANT players to our guild that might be on an extended break. (I being someone who left the game for 8 months then came back to be the leader of one of the more successful guilds on our server.)
That would stop the "zerg" and still be able to not have to delete every toon when they haven't played in a week.
500 toon can still easily translate into 1-2 members on a month in many, many dead guilds. If you don't constantly recruit, even with 500 toons you will soon die.
We have had more than 250 of our toons log on within a 24 hour period on a Friday not too long ago and we often have as much as 250 active accounts (logged on within the month you can see it on the guild manager). You can't have that if you don't constantly recruit because one by one they will fall away to other games or real life and then you'll be left with one of the probably HUNDREDS of guilds that exist with near 500 toons and no one actually online.
Guilds make all MMOs, the guild limit is prohibitive. It needs to allow for people to play their alts in the same guild as their mains without having to kick someone who might be away from the game.