I agree that would be ghastly if even 10% of the MLS agents did it - we must have several 10's of 1,000's in mine (now THAT I hate).
But, I've dealt with enough agents and buyers and some WANT to know if I come across something such as "A" or "B" before it hits the market - could I let them know - type thing. If I do, I notify them, and, I've reversed the situation with them also.