Actually, transform feedback is already supported on both Sandybridge and Ivybridge. However, we'll need this in place to support geometry shaders (3.2) together with transform feedback. It'll be useful for TF + primitive restart as well. Right now, we count some things manually in software, rather than relying on the hardware counters; this lets us use the hardware as intended.
Also, I feel compelled to mention that Ben Widawsky deserves all the credit here - I just made the couple line Mesa patch to use it in a fairly preliminary fashion. (Plus it was wrong, and Ben had to fix it.) I'm definitely grateful to Ben for doing this.