I think it's a measure of experience which can relate to skill level. As you say, the more experience one has at something, generally the more skillful they will be at it. There are exceptions but there is no other way to differentiate good players from bad, that's about all you can go by. Of course you could research every player that may want to join your fireteam but that's not very practical. You can ask for whatever you want in a fireteam member. If some don't like it, too bad. So the bottom line is if you want to use it that way, I have no problem with it. Many would have a problem with it but it's their problem not yours.