That’s an interesting question; it actually seems like most pros these days are playing new guitars like Collings, etc. rather than new Martins. I don’t think it’s because all Martins are just worse, I think it’s because Martins are generally less consistent from guitar to guitar than some other brands. I’ve heard/played some newer Martins that sound amazing, and also some that I preferred my $900 Eastman to (Of course my Eastman is stupid awesome which helps).
Tony Rice said in his biography that “people are playing guitars other than Martins now because Martin ain’t made a decent guitar since 1955!” I would guess that’s a good amount of exaggeration, but it is true that most would have way more confidence buying a guitar like a Collings without playing it first than buying a new Martin without playing it first. Also, some people who own incredible Martins from the prewar era might choose to jam with a newer guitar from another manufacturer so as to prevent damage to their more valuable instrument.