The RQ-170 landed more or less intact. And in the case of the F-117 it was useful to examine the materials used in the structure of the aircraft like the airframe and the radar absorbing paint. Knowing those characteristics would help them refine a radar model of the F-117. It is quite clear the US thought the design was compromised since they removed it from service shortly afterwards.
What I saw of those Flanker wrecks would provide little data on the most relevant aspects of the Su-35 like the avionics or radar. You would be examining little broken pieces you got with tweezers.