Rangers are already fighting the Imperial Legion. They are already hunters: how would you have an entire other guild that does the exact same thing and still offer unique gameplay? Also... Knights? Robin hood Guild?
Oh they wouldn't offer the same things, more like "player have to make a choosing to get what he wants the most". Don't worry about the names, they're simplified for identification of their "style". I would hate it myself if something was named Robin Hood Guild
* Fighters Guild vs Knights Guild: Fighters are the local militia and fancied by commoners, knights are the organized ones and liked by the wealthy.
* Mages Guild vs Whisperers Guild: Mages are the "low yield" entry stuff found in cities, whisperers could be necromancer lairs or those doing dark magic.
* Thieves Guild vs Robin Hood Guild: Thieves how they were in DF (no morals, deal with freelancers), Robin Hood what they became in OB (steal from the wealthy, located in wild)
* Morag Tong vs Dark Brotherhood: Organized and legalized vs the sheer Brutality gang.
* Hunters Guild vs Rangers Guild: Hunters hunt for food and that's basically it.
So if you join Fighters Guild, the Knights Guild would hate you and not let you in and vice versa, as they have issues with each other. Typically the first one listed is what you might expect to find in towns. By joining them you may be on better standpoint with the locals, but the reward from them might be kinda small. Join the right one for better rewards, but it may be harder being liked by the poor and commoners.
See more here: http://www.gamesas.com/index.php?/topic/1168935-guild-ranks/page__p__17221325#entry17221325
Ignore the quest numbers, was just illustrating the use of exponential growth to make it harder and harder, requiring true devotion.