Actually there is a glut of quality players still on the market. Most of them are past 30, like Kelly, but there's no reason to think they can't contribute. John Abraham, Shaun Phillips, Dwight Freeney, Ahmad Bradshaw, Seymour, Antoine Winfield, Sheldon Brown, Woodson, Brandon Moore, all of those guys can still play and they're still out there.
The Pats this year have limited draft capital, a fair amount of cap room, and relatively few holes on a mostly young roster. This is the ideal time to shore up the roster with low-cost veteran signings.
A signing like Kelly is important because it gives them some breathing room for the draft. Before adding a quality vet on a two-year deal, the Pats would almost have been forced to draft a DT high this year, with Love and Deaderick hitting FA next year. Getting Adrian Wilson for three years did the same thing for the safety spot. It may not solve the problem, but it makes it less of an emergency.
Also, there are a lot of teams out there that aren't close enough to contention to spend any money, even 1.5-2 million, on a 32-35 year old player. The Pats being so close makes these signings justifiable and gives them an advantage. They can spend real money to upgrade the veteran depth on the team, whereas the Jets or Bills really need to be investing on upside only.
That's why this portion of free agency should be the sweet spot for the Pats. They should be able to pick up 2-3 good-very-good veteran players for low dollars, and it's guys like this who could make the difference next year -- they were just a couple of players away last year, so it seems to me this is a critical period for them. If Kelly can still play, it could be a pretty big deal.