Sure, Rivers may have taken a Riewoldt etc before, but I wouldn't say he is a CHB/FB. Of course - as you said - it all depends on definition, and there's been some debate on these boards about whether Rivers is a CHB or not. Daniher generally starts Rivers as the 'third tall' unless we're in desperate trouble either during a game, or at the selection table. Thus the assertion that he's not a key defender...
Anyhow, I think we can all agree that whatever he is, he doesn't do a bad job of it. As you intimated, Whelan is hands down our best small defender.