The natural question is: if you're just paying to cut in line but you go through the same security screening, what's the purpose of the background check and the biometrics? One could argue, I suppose, that once you know that people were OK, you could give them lighter security screening, but as far as I know that's not what happens: TSA only has two security modes: normal and aggressive (SSSS), but it's fairly easy to avoid aggressive mode with a boarding pass printer, so it's not like you need any system this heavyweight to securely exempt people from random selection. The cynical might argue that the purpose is to protect Clear's ability to extract money from you by preventing you from giving your card to someone else. On the other hand, you don't really need a thumbprint, let alone an irisprint, to stop that. A photo would be plenty. And of course the background check is totally unnecessary.
I suspect that the real reason here is that Clear was originally conceived as a bypass system where you would be able to get lighter (or perhaps no) screening, and in that context the background check made sense. That didn't work out, but the initial security theatre stuck around. After all, how would you explain that it was somehow no longer needed?
