For one simple reason.
My experience from other 'freemium' games I've played always ends up the same.
At first we get to see a few 'top guild wars' and you get recruited by higher ranked guilds and so on; good fun and all.
Exclusive rewards can only be obtained by ranking top guild.
And then we have the final result:
Top guild is for whales-only with 50x the points of the 2nd ranked guild.
Then you have the next 4 ranks who are also in a whole other category than the the 6th rank, so basically all FTP players will eventually realize they'll never get any of the exclusive rewards and most guilds start falling apart...
"So don't join one then" you say. But the sweet rewards!!! Plus the team-work aspect of it is enjoyable. Until it isn't.
It's like being given a sample of cake, then you get to watch fat pigs tear thru it and eat it all while you drool and stare blankly....
Bah, this whole post is kinda pointless.... But still. I fear a huge mass disappointment coming if guilds are introduced.