Ok, I'll take a moment of my time to rant about the those don't like gearscore and achievements are required to do a pick-up raid.

These are the conventional arguments:

1) lolol achievements don't mean anything, just because a player doesn't have achievement, doesn't mean they're not a good raider or don't know tactics

2) lolol gearscore doesn't mean anything - I have seen lots of people with high gearscore who don't know how to play

First of all, I'm not actually disputing the claims in those statements, but simply their relevance. The thing is - pick-up raids are about clearing content fast that is considered to be farmable. This means to some extent outgearing the instance and/or already being trained in the tactics applied. Secondly, pick-up raids are about getting the job done - thus minimizing the likelyhood of wipes and spending time explaining tactics.

We can also assume the following:

1) While an achievement doesn't prove that you're a capable raider in the instance in question, there's a greater chance that you are, if you have the achievement, than if you don't

2) While gearscore doesn't prove that you know how to play your class, there's a greater chance that you are able to play your class, if you have decent gear.

3) No mechanics exist in game to measure skill and knowledge - gearscore and achievements are as close as it gets without individually evaluating each player, which would take too long.

So the reason behind the gearscore and achievement inspects (a good PU raid leader will inspect gear/gems/talents as well as achievements) is to minimize the chance of failure. Even though you may think it's unfair that you, the greaters raider since sliced bread (despite you not having done any raids and your 3500 GS) , get passed up for someone who has done the raids before and has way better gear, but is a poorly skilled raider, it's really not. The reason? The PUG-leader has no way of knowing about your awesome skills and you have no way to prove them.

TLDR: Gearscore is among the least sucky tools to evaluate player without performing a time-consuming individual analysis / chat.