Player's perspective

I remember how I hated when people wanted you to link epic achievement, since seen it as pointless, since it doesn't say anything about their current gear. With new tiers, it became meaningless, and now people use addons to measure how geared you are, and check your current gear, the one you will use on the raids. Of course it is far from perfect, since it speaks only about the level of gear and not your choice of gear.

But lets see the whole thing from a different perspective. If you were unwilling to do enough heroics to get decent badge gears, since you give up when you are bored and doesn't like to work for hard earned victories, how could the rest of your raid trust you? Why would they think: You will stay when they are saved and you wipe partly because of your weak gear and consider the raid too hard and too boring with wipes? How would they expect progress with you?

As you see the bare minimum for a raid is respect for all members. You should respect their effort, you should respect they have put hard work into the chance of success, and you should respect them by saying: I should carry my own weight, I can come just as prepared as them.

High gearscore doesn't mean anything but time spent on character and isn't interesting. But low gearscore show one thing: You didn't like the effort to get badge gear, and you didn't work hard, and you doesn't respect the efforts of the others much. And for this reason you shouldn't be invited.

While high gearscore doesn't mean anything, low gearscore shows: You didn't want to prepare for raids, you didn't and doesn't respect other raiders effort. Why should they take you then?

Gearscore isn't a tool to ensure success. But it is a tool to keep some of the worst players out of raid. Worst not because skills or gear. But because they don't respect the efforst of others, and due lack of this respect you cannot trust them. You cannot trust them to try hard, you cannot trust them to stay when they got their loot and the raid becomes hard. You cannot trust them, since trust is based on mutual respect.

And you shouldn't team up with people where the basic respect and trust is missing.

While for Naxx, OS and many other raids Gearscore requirements seem to be insane because people who meet the requirements would be far overgeared, but doing your best for raids will mean having at least full badge gear both for naxx and for progress raiding.

People will trust that you don't cheat at gearscore since you want good chances, and if you had patience to get good gear and put effort into it, you want the best outcome at raids and want to use your gear and skills at best of your knowledge. They can inspect you, check your statistics at armory, if they want to know more about how you choosen talents, gear, etc.

But they know that raiding is a team effort, where everyone should focus and try their best. Both while doing the raid, and while preparing to it.

And for them low gearscore is a clear sign of someone not wanting to be a member of team with equal responsibilities. It is a sign of someone who thinks (s)he shouldn't work as hard as others, and others compensate for his lazyness. So much for teamwork.

Gearscore can't measure how good you can do your job, how good your choice of gear is, but can show how much hard work you put into making the raids succesful, and that you are willing to work just as hard as the other members and they won't have to carry you as dead weight.

It measures time and effort, and with this respect. Not your gear, nor your skills. Respect and effort you put in to reach goals of a team are qualities that can speak a lot about you and can be used to judge you more fairly than any DPS meter. For this reason, GearScore is a good addon.