Others have touched on some points but my two main ones are:
1 - Forming a pre-made can be time consuming. My experience with pre-mades has been poor. Maybe it's just my luck but when it takes 30 mins to find the right mix of people and I only have an hour, I would rather just roll the dice and PUG. Some would say, if you are in a guild, forming a pre-made is easy, which brings me to point 2
2 - I am not 40 yet with any of my toons and all the guilds that have been recruiting want 40/40 players with gear, etc. So I am guildless which brings me back to point 1 and why I PUG.
Also there is a world of difference between a WB PUG and a SC PUG. Most of my WB PUGs have been great as long as the WB Leader knows what they are doing (and people listen). Those leaders have text macros, which helps out immensely and I have had little issue. The biggest problem with a PUG WB is people tend to just jump ship when things aren't going their way. Sucks when you are at even numbers in a zone, the enemy gives you a good beat down once and suddenly 2 full WB of people leave, ensuring your next zerg encounter will be another loss.
SC PUGs are another animal altogether. It wouldn't be as much of an issue however if the system was a bit more picky about balance. I've mentioned this in other threads but if the game waited for just 1 healer per group it would help quite a bit instead of just filling the positions with 12 random people. Healing is king imho and if you have 1 healer and the opposition has 3, the group with 1 healer is going to lose 9 out of 10 times. That's pretty much my only problem with SC PUGs (that and getting a full group of nothing but SWs

, happened to me once, was funny but horribly sad at the same time)