I think the notion that all small guilds are using phantom accounts is absurd. Our small guild originally consisted of 5 real life friends and a few others. We do have one person with 2 accounts in our guild but everyone else has one account / person. Whenever someone asks me to invite another character I check whether our account # goes up. There are many people that have multiple ftp accounts because of the character limit and to make it easier to trade to bank characters. The person I know the most accounts is in a mid-size guild and not a small guld.
The system certainly encouraged a guild to be at least 6 accounts, but all that really did for small guilds was to reduce the significant per account penalty it already had, it never gave a leveling advantage.
The biggest advantage a guild could have is to have all active players regardless of size. I realize this is definitely harder for large guilds to accomplish than small guilds, but it's not the reality for most small guilds either.