Reviewing the possible reasons why non-Nikon lenses performed so poorly in my recent shoot-out, I came to realize that the Nikon D90 will activate only the Matrix Metering II metering method on Sigma, Tamron etc. ie. lenses that are not made by Nikon!
The 3D Matrix Metering II engine is reserved only for Nikkor G/D lenses according to Nikon official specification chart. Hence, possibly, if you shoot in purely spot or partial metering mode only, the result with the use of these cheaper lens may turn out slightly more predictable and perhaps better?
As for me, I am sticking with Nikkor-only lenses with my Nikon D90.