Trading down in the draft for more picks always seems like a good idea unless those 2nd and 3rd rounders turn out to be busts.
The best thing the Patriots have done with some of those lower picks is turn them into veteran acquisitions. They have been successful at FA acquisitions and trades. Vrabel, Harrison, Moss, Welker, Dillon, and the various OL FAs they got were an impressive group of FA acquisitions/trades. I don't think you can compare what the Patriots have done draft-wise with a team like the Steelers who have built multiple championship teams through the draft.