Numbers alone are just not by any means a good method of judging someones future production, when you make trades you don't get the #'s they've earned up to this point you have to forecast, and big names are a lot easier to forecast and give the most confidence in that forecasting. When Brian Roberts hit 8 HR's in the first month of the season last year, it was completely out of rightfield, and if you traded for him based on the idea that maybe he'd hit 30 for the year you'd of felt quite foolish at the end of the year, even if what you got rid of was a struggling big name.
You never quite know when guys who aren't considered developing stars are just streaking or coming into their own, so unless the guy without the big name has a strong potential filled pedigree it's going to be hard to trade them for big names, even if thus far they've out performed him.
You have to take into account not just current production, but what's the max production you think you can get from a player. You have to weigh the likelihood that said player will continue on this torrid pace to the idea that said struggling player will continue to struggle. In a Victorino Ortiz situation, Victorino would have to continue at the same exact pace or better and Ortiz would have to continue this underachieving pace. Especially in someone like Ortiz's case when he's obviously still seeing the ball and a breakout just seems like it has to come considering he's still hitting for a very good OPS, you'd figure eventually the power has to come.