At first I thought this was left from the previous owner, but after watching a Road test from Motorweek, their test car had the same ugly antennae.

So i'm guessing it'* for the On-Star? I took the anttenae off and it looks much cleaner.

Haven't noticed any changes.

yeah, its for onstar. my mom'* trailblazer had it.

yes that is for the ONStar. You could remove the entire mount if you wished. IF your Onstar is not active, there is no change in the operation of the car