I'm not sure the Bills are building for a 3-4. They are in a 43 most of the time and the Merriman experiment doesn't seem to be a long term fit. That leaves Batten, Moats and Kelsay as the depth for the future. So unless the Bills go OLB x 3 in next years draft, I expect them to a 43 hybrid team for a while.
Plus, KW and Dareus are natural 3 techs. If you don't want to take my word for it, take Warren Sapps (he might know a thing or two about the position) or Mike Mayhock who both have said that on multiple occasions.
So if Osi isn't the greatest run defender, neither is Kelsay or Merriman! But you know what he does do, get after the QB (2 sacks today). And that is what this team needs. Plus if you improve the pass rush, you can drop a safety down in the box more consistantly to help the rush D.
Like I said before, I wouldn't give up a ton for him but if he's still available, I'd see what it takes. Calling him the same player as Kelsay in asinine...but that's Simon being Simon.