The spots are full in the sense that there are two veteran starters and three young guys with a lot of upside. No point drafting another WR when you've got Edelman, Tate, and Price waiting to prove that they can play.
I don't have the exact numbers, but I'm guessing the 2007 team carried more than 5 WRs. That was a 5-wide, shotgun offense. In this 2TE offense, the 3rd receiver plays just under half of the snaps, and the 4th would have been closer to 12% or so if not for Welker and Branch getting week 17 off.
I'm all in favor of getting an upgrade, but adding more guys to the back end of the depth chart (in the form of rookies who need to learn the scheme) is overkill.
Increased roster size is a good point though. If we assume an extra 5 spots or so, I think in general we're looking at an extra offensive lineman, two defensive lineman, and a defensive back, since those are the guys who tend to get banged up a lot. In an 18-game season, that's what is going to dictate those extra spots. Might be room for another WR, but I think that's a position that is already overstocked by pretty much every team, since you can put 5 on the field at once, but very few teams ever do.