This is total speculation. But it's obvious the Bucs have to address the quarterback position. If they truly are ready to move on from Jeff Garcia, a distinct possibility, and they aren't ready to crown Luke McCown their starter, then they've got to do something. Both those players are free agents anyway.
It will be expensive to acquire Cassel, especially this year when the quarterback market both in the draft and in free agency is very uninspiring. Allen, for example, netted the Chiefs a first-round pick and two third-rounders. But this is not a defensive end we're talking about. We're talking quarterback here, and a potential franchise quarterback, at that (when was the last time those words were used in relation to the Bucs?). One reason Tampa Bay, if inclined, is positioned to do this deal is its massive amount of salary-cap space. The Bucs can give Cassel the big-money, long-term contract he would be seeking without breaking a sweat.