There's a lot of content packaged for the AUR that isn't readily available to people using less enlightened distributions.
I use Arch BTW.
Seriousposting: a lot of software just isn't packaged as deb or rpm because no one has taken the time to do it. The AUR is really fucking convenient as an end user. And yes, you should always skim AUR packages to be sure they're doing what they claim to do and aren't garbage, anyone can post anything. I have seen a lot of trash uploaded to the AUR.