I expect it's because Javon Walker is damaged goods. The third round pick is to sweeten the pot.
Santonio and Chad Jackson are supposed to be the best two receivers in the draft. I expect Holmes will be of more use returning kicks than as a number one receiver, at least for the first half of the season.
On the other hand, Terry Glenn, as a rookie from Ohio State, and Torry Holt, as a rookie from NC State, both contributed alot right away.
I don't mind this trade. Which probably won't happen.