Garza was pretty dominant in Triple-A last season. The innings total wasn't that high, as he started the season in High-A, but he was drafted as a college pitcher and is more than ready. With his velocity, pitching in the minors might be counter-productive. Both Garza and the Twins have talked about him focusing on his breaking and off-speed stuff, because even at Triple-A he can just blow 97 MPH fastballs past hitters and doesn't have to use his secondary offerings that much.
From what I've read, Garza just wore out at the end of last season. His innings pitched per season for the last three years are roughly 90 (NCAA), 160 (NCAA & Minors), and 180 (Minors & Bigs). The Twins have done a good job of ramping up his innings over time, and Garza should be just fine this season, but he's got nothing left to prove at Triple-A and should be in the Twins rotation. He's their third best starter, assuming he doesn't blow past Bonser this season, which could happen.
I forget which Twins blogger said it, but the quote was something akin to, "What does Garza have to do to win a spot in the Twins rotation? Bounce around the majors for ten years?"
The Twins are paradox. Terry Ryan never winds up on the short end of a trade, and the Twins are hands down the best organization at scouting and developing pitching. However, a lot of their recent hitting prospects have flopped (Moses, Span, Plouffe) and they have a sick fascination with mediocre veterans and do not trust the talent they develop themselves. This is why Twins fans had to watch Jason Bartlett hit .300 in Triple-A for the third year in a row as the team broke camp with Juan Castro as the starting shortstop last season.
Silva, Ortiz, and Ponson are the 2007 versions of Castro, Batista, and White (the problem is, the Twins also have a 2007 version of White).
At least Gardenhire and Ryan have shown a willingness to change course during the season. The problem this year is that they probably aren't going to get anyone in their minors to come in and produce like Liriano did last season, and on top of that, with the retirement of Brad Radke, are another pitcher short in their rotation.
Perkins and Garza should be in the Twins rotation right now, and there is nothing wrong with Scott Baker as a fifth starter until Slowey, Sosa, Durbin, Swarzak, ect. are ready. Combined, Silva, Ortiz, and Ponson will make a shade over $9 million in 2007. For a team that hopes to contend with Nick Punto at third and Rondell White in left (adding White's salary to the $9 million would give you $11 million), and prays Jason Kubel will stay healthy at DH, it's money that could have been better spent elsewhere.