I recently attended an Alabama Symphony Youth Orchestra concert which did this, presumably because the younger audience was assumed to require additional interaction to alleviate boredom. For the "grown-up" concerts I've seen lately, about half the time the composer of the featured work is in the audience or on the stage as a performer, and will offer comments. Pre-concert lectures are also common, although I'm never early enough to attend those.

I do love symphony concerts. I'm a little sore at the moment because the ASO's music director just announced he's leaving his position next year, and he's been truly wonderful. I hope they can attract someone just as talented to replace him.