Wipe aversion is actually very understandable in PUGs. One of the key differences between a PUG raid and a guild raid is that a guild raid is actually pretty certain that they are going to win. Maybe they won't win tonight, or even this month, but raiding guilds don't let that dampen their enthusiasm. When the members of a guild lose faith that they will ever beat an encounter the guild often starts to fall apart. A single boss can not only wipe raids, but it can outright destroy guilds, if it is capable of making people feel that no matter how hard they try and no matter how much they practice they will never win.
In a PUG, winning is far from inevitable. You really might just plain lose and walk away with nothing. It's a potentially disheartening affair, and at some point continuing to fight the boss is throwing good money after bad. If you no longer think the raid is going to succeed, then you should bail and do something more productive with your time.
Much worse than this from a raid cohesion perspective is that once one person leaves, the threshold for another person to leave plummets drastically. After all, the first person has to be unwilling to sit through one more attempt. The second person has to be unwilling to sit through the time it takes to replace the first person plus the time for another attempt, and so on. One person leaving - especially from a 10-player raid - can be enough to set off a cascade.
Obviously no one in a raid wants this to be the outcome, but people running the raids have an additional investment. The majority of people, even people who seem like jerks most of the time, actually have an internal sense that when they start something and it fails, they deserve the blame. That feeling of shame and blame may be warring with other feelings - such as rage against the group's lousy dps - but people mostly don't like to fail and as a raid leader you can't really leave the blame with other people. If the group failed, it's because you put together a bad group.
Unless, that is, you put together the group based on an objective measure that is supposed to tell you whether people are capable of succeeding or not. If you go into 10-player ICC with a group wearing ilvl 264 gear, then you "should" win. If the group can't beat Saurfang and degenerates into name calling then it is pretty easy to say to yourself that the members of the group must have just been bad players.
Because the tool is objective1 the raid leader doesn't have to shoulder the blame for its outcome. The tool really becomes the raid leader; and given that PUG raid leaders rarely explain fights, go over what happened when there is a wipe, give motivational speeches, or talk to individual raid members about how to improve their play, the tool might as well be the raid leader for all other purposes as well.
None of this is to say that PUG raid leaders are wusses or to put the blame for anything on their shoulders. Very few people decide lead PUG raids precisely because being a leader carries with it a burden. Moreover, people who are really great raid leaders are mostly raid leaders in guilds. The skills it takes to be a great raid leader aren't really that different than the skills it takes to be a great leader of people in general. Those skills are not common.
Blizzard thinks that implementing an Raid Finder system similar to the Dungeon Finder system is a problematic idea for a lot of good reasons. GearScore, however, serves as a makeshift Raid Finder that players have created themselves. Using an objective entrance criterion that should theoretically be tied to the group's chance of success takes the raid leader and their responsibility out of the equation.
In an environment where a single wipe can lead to a complete failure, you'll get very few volunteers to shoulder the responsibility for the success of the raid. The number of PUG raid leaders who want to use GearScore to determine raid inclusion is just a consequence of that fact.
1. It isn't really that objective because you have to decide what GearScore you are using as a minimum. My feeling is that most raid leaders choose values that are well above what is really needed to do the dungeons and rely on overgearing rather than appropriate gearing to beat content. Of course they do this for the same reason they use GearScore in the first place - to try to minimize wipes.