Not really, actually I am only in 3 guilds atm because I quitted 2 inactive ones. In other games you can usually join just 1 guild so it was nice surprise to me that eso allows up to 5. Sometimes I'm wondering to join another trade guild, but it's not really critical atm. Especially when other guild I'm in occasionally wins some guild trader too
So I don't think we need more, 5 is more than enough. People who wants more are just a bit spoiled. If they increase amount to lets say 7, then people will ask after a while for 10 and so on.
I'm currently in 3 guilds atm (1 for trading, 1 for groups, and 1 for casual stuff). In fact, 3 guilds is the most I've been in at one time, come to think of it.
"If a person is truly desirous of something, perhaps being set on fire does not seem so bad."