Why this makes sense
If the Buccaneers love Johnny Manziel, trading up for the player they view as their franchise quarterback is perfectly logical. There's no one like Manziel in this draft (or any draft, really), and it seems unlikely that the living highlight reel would fall to the Bucs at number seven. So a trade up for Manziel is mandatory, if they want him.
And wanting him makes some sense, too. Defensive coaches always talk about how difficult it is to defend Michael Vick, or any other multi-dimensional quarterback who's an outstanding improviser. They even say this when said quarterback isn't consistently productive. It's certainly possible that Lovie Smith fell in love with Johnny Manziel after watching him blow up college football this past year.
The Bucs would likely look to limit Manziel's randomness in games by relying heavily on the run and a quick passing game, which would allow him to get the ball out of his hands quickly. Meanwhile, they'd try to harness his improvisational skill and intuition on pass plays that call for longer dropbacks.
Why this doesn't make sense
The fit of Johnny Manziel with Jeff Tedford's offense, which thrives on structure and predictability, is dubious to me -- although the Bucs would know that better than I ever could. There's a certain tension between an offensive coordinator who tries to find an answer for everything, and a quarterback who has a tendency to abandon structure for improvisation.
In addition, the Bucs don't really have the ammunition or roster to be able to afford trading up for any player. They lack a fourth-round pick, and have to find at least a starting wide receiver and a starting guard in this draft. Trading away picks won't help them get that done, and doesn't fit with the Bucs' apparent preference for trading down.