We've compared Roth to Ninkovich.
He excels at setting the edge. He had the most tackles of any of the free agent 3-4 OLB's we've considered (86 compared to 62 for $72mm Charles Johnson). However, he had about the fewest sacks with 3.5.
So, as people point out, he doesn't offer much pass rush. He doesn't even replace TBC.
He does offer sorely needed depth and experience in the 3-4. He's pretty stout - 25 pounds heavier than Manny Lawson. He doesn't seem to cover a whole lot.
The Pats run defense, with Wilfork and Haynesworth, Mayo, Spikes, Roth and either Cunningham or Ninkovich, would be very solid. None of those guys have delivered a lot of pass rush.
No one will compare him to McGinest, Colvin, or Vrabel. Adding him enables a rotation of Roth, Ninkovich, and Cunningham - with the possibility of Carter or Moore. Top three would be pretty interchangable. Roth as a starter is certainly not a major downgrade from TBC (as much as TBC started).
In summary, based on where the Patriots are, this spot is their only glaring hole, Roth plays the position, is available, and shouldn't be terribly expensive.