This is definitely a trade with its pros and cons.
On one hand-even with Holliday not producing like he did in Colorado, he's still a huge upgrade in LF over the duo of Chris Duncan and Rick Ankiel. Good baserunner and defender. Gives Pujols protection in the line-up. He has been hitting well in July, and maybe a return to the NL and a friendlier hitters park will help him return to the close to MVP form of 06, 07, and 08.
On the other hand-they gave up a lot to get him. Wallace projects be an impact bat(there are questions about his playing 3B, he might be a 1B instead and that position is sorta blocked in STL). Mortensen could be part of a starting rotation. Don't know much about the OF they gave up. This could only be a 2-month(and hopefully playoffs) rental, and if they do re-sign him it'll take some big money(though they would of course get compensatory picks if he signs elsewhere). Holliday is a Boras client. The last time the Cards made a trade with Oakland they definitely got the worst of it, the Mulder for Haren, Calero, Barton deal. I imagine a rotation this season with Haren, Carp, Wainwright, Lohse, and Piniero and it makes my head hurt.
I have a feeling I'm gonna like this deal this season, but regret them making it a few years down the road. The pressure is definitely on for them to win now. They have a lineup that can move up from the middle of the pack with-
2B Schumaker 3B DeRosa 1B El Hombre RF Ludwick LF Holliday CF Rasmus(and probably Ankiel some too) C Molina SS Ryan
A good top 4 in the rotation that could win in the playoffs with Carp, Wainwright, Piniero(I don't know whats gotten into him but I hope he keeps it up) and Lohse. And the bullpen has been very solid with Franklin taking control of the closers role.