First of, this like asking why there are rbg guilds and arena team guilds.
From one prespective, you need something to represent. You feel more pride knowing your representing someone. For me its representing both my guild and realm.
Being in a guild, you feel that you truely "belong" with the people you enjoy pvping with. To be honest, have you ever been in a worthy guild full of skilled players you know you can rely on?
And of course funding. We have many people my guild who are amazing at pvp, voice com, clutches, etc. The problem is that they were all f2p before and are a bit clueless on how to gain gold, so we fund them! Sure you can look at tips and find easy ways, but then that requires work and many annoyances. Factors like server economy and shit come in play. Why do all that when you can reroll and get say shadowfang and all your enchants instead of spending all your sub time farming gold or even the item itself. You wanna enjoy yourself with no hastle.
And as lenny mentioned wargamesss
Which btw Needs Mastery vs Nagaz with Attitudes comming up soon
Its just overall nice to fall back on a guild
As for nasty insulting players.. that sounds like a personall issue
And who wouldnt want to have a special bank were all your twink friends can access and be full of rares BoE, free repairs, and not to mention the guild buffs