When teams - even the Patriots - misuse the Franchise Cap, I don't think its good for the team, or to football in general, or fair to the player.
I think with Brady's health being questionable, using the tag on Cassel is justifiable - and if someone makes them an offer that has the Patriots decide its worth the risk to go into the season without him so be it
I'm sure the Patriots front office will be doing their best to keep word of Brady's health as secret as possible.
None of that changes the fact that Schaub and Cassel's situations have some pretty significant differences in the leverage that the Patriots hold, the leverage that Cassel holds, and the leverage that an unnamed team holds in calling our bluff on whether we're prepared to pay $14 million for a season, complicated by the fact that we'd be looking for them to take on an expensive salary and give up a 1st round pick minimum.
As I said - it's more complicated than answering "yes" to the question of whether he's worth a first round pick.