I believed that this is already suggested in the past. If I remember correctly, one of the admins say that you can make a faction without a plugin or something along those lines.
And besides, if the MCWB add more worlds to the server, then the entire server will lag more. And upgrading/buying a server is expensive.

Yes, I agree with Fuzzball. Though this may seem like an awesome idea with teams going at it in various warzones. I think it would come to just a bunch of different one or two manned factions instead of a few large groups.

I play on a factions server for a video series I do and they are always looking for new players. I won't post it as it's not a family friendly server but if you message me Aldaz, I'd be happy to give you the address.