Quality of pug groups deteriorates over time. If you pug it early, generally the later tiers of content will be populated solely by pick-up members who know their jobs and the fight well. As time goes on, more and more players are able to overcome early tiers by force of gear ilvl, or manage to tag along with better players. They start to pollute the pug pool, and it becomes less and less likely that you'll be able to put together a pug group that can defeat the final tiers of content.

At least, that's been my experience. I'm still amused by the whole "SKIP SOAR" debacle with Zurvan. By the end of the patch, you'd have to be mighty lucky to find a pug group that could skip soar! Skipping soar was the venue of the quality players who won first and farmed the crap out of the fight before leaving it forever. No, by patch's end, you didn't hope for a group that could skip soar - you hoped for a group that could HANDLE soar.

A static is always preferable, in my opinion, but creating and maintaining one can be a nightmare. It's HARD to coordinate meeting times for eight skilled people, especially when those people have real-life schedules that change from time to time, unexpected events that pop up at inconvenient times, and so on and so forth.