The T-X always seemed like a step back in the evolution as well. It's endo-skeleton limited so much more to it's pouring and squeezing through areas.
The T-1000 is a prototype... which suggests its technology is meant to be applied, not fielded as it is. My personal belief is that the T-1000's mission was a field test.
Whereas the T-X was built for a specific purpose - to terminate machines, hence it's arsenal of inbuilt nanotech weapons, combined with mimetic polyalloy as an exoskeleton.
T-1000 is basically raw technology...
What's the purpose of humanoid Terminators from the 600 series onwards? Basically infiltration units - disguised with the intended ability to blend in with humans. The evolution actually makes a bit of sense:
T-600: rudimentary endoskeleton, ineffective rubber skin
T-800: refined endoskeleton, perishable flesh disguise
T-1000: prototype technology fielded as a functional model
T-X: advanced endoskeleton, in-built weapons, applied mimetic polyalloy skin - almost infinitely versatile and restorable disguise
And no, I don't think the concept of mimetic polyalloy is that far off... it's basically nanotechnology infused with alloy, each atom a thinking, working machine that communicates with one another, actively changing rigidity as required.
Damn, I feel so geeky...