I do this at any gig. It is usually the very first thing I do. Having grown up a drummer, it is my instinct to be a gear-head. It is a great way to get to hear things in real use. Much better in my opinion than going to a store and playing stuff there. I also get to see some interesting set up ideas....many guys breaking the TB rules and using mixed speaker cabs (4x10 and 1x15

) , guitar effect pedals on bass

; boutique basses in metal

; 4 string P's in jazz/funk

....
the one thing I notice I DON'T do anymore is judge a guy by the gear. I used to do this almost exclusively. Now I wait to hear it, and then make the judgement.