We should be concerned no matter who our back up QB is if Tom goes down. Unless of course we can somehow get Joe Montana and stick him in a time machine. But Polian probably as a rule against that...
Anyway, where was I? Oh right.
I like Cassel as a back-up overall. About the third string QB thing, anytime your one and two quarterbacks go down you are pretty much screwed anyway. That being said, I wouldn't be oppossed to bringing in a vet, maybe like Testaverde, but 87 is old. Back-up QB is a funny thing. It can either be completely irrelevant or the most important position on your team. But I don't think a third string quarterback is going to make or break your team.