I don't think Thane is wasted too much in ME2... it's a suicide mission and you're gathering the best of the best. He fits in that scheme.
He didn't get a role in the SM other than guy with gun. Least important ME2 team member.
-doesn't work as infiltration/tech specialist
-doesn't work as squad leader
-doesn't work as biotic bubble generator
-doesn't have a particularly good defensive rating when holding the line
-doesn't provide any of the 3 crucial ship upgrades
Every other squadmate than Mordin (who was central to countering the seeker swarms earlier in the plot) do at least 1 of these, some do two or three (Garrus, Tali, Jacob). Thane's just some assassin, but how is that particularly useful for fighting bug thrall things and breaking into their base? He can't go snap Harbinger's neck or poison his food. They should have actually given him something to do IMO.