"Sets" bore me to death. It's like the Rogue is just "using" her co-op partners to handle the things she can't (or won't) snipe. You're right -- it doesn't take any skill on the part of the bucketheads. It may be the "safest" route, but only the Rogue gets to shine.
Whenever a bad melee broke out in our IM game (I'm thinking of dlvl 3 vs Horrors) and we didn't have a set, I remember our faithful Bard getting dropped to ~10 hps. The missing co-op skill: If the tank backs away, the "axe" scrambles for safety.
Like you said, co-oping skill must be developed. You can't do that by "using" your co-op partners. Mechanical sets act; co-opers react.
[o: *LEMMING* :o]