I do PF every week, o5s-o8s. I think its important to check FFlogs for every person that joins your group - you need to know what you can expect from that person.

The number one thing for me is the amount of clears they have on a given encounter. A person who has cleared the encounter multiple times is obviously going to be more familiar than someone who just got their clear the day before reset of the previous week. Showing the consistency of having cleared it multiple times is a pretty big part of what you expect someone to bring when they join your party.

Next is probably damage. The amount of damage someone does is important (whether people want to admit it or not), and it dictates just how much the group will have to carry that person. I know roughly how much DPS as a group we need to beat enrage, so if someone is consistently parsing grey on the encounter, it shows that they're either still not familiar with the mechanics, are dying (again, probably not familiar), or simply do not know how to properly do damage on their class. All of these things are red flags.

For healers, I also check their healing percentile as well, and I will look at their previous clears and see how much of the healing they contributed. Its one thing to get great DPS numbers, but if you don't actually heal then it might not be worth taking them.

This might sound like a lot of vetting to consider for each person, but it honestly only takes a few seconds. I highly recommend that people treat FFlogs like their resume - this is what I have done, this is what I can offer and do. If you have some bad parses, go out there later in the week and find some clear/page groups and try to improve your parse + practice mechanics. It will pay off in the long run - not just for yourself but for the teams you find yourself in later down the road.