I only switch racquets when my grip is too sweaty, or if I need less/more tension. But all of my frames are the same make/model, so I get no benefit in that regard.
I can't stand having different brands in my racquet bag. It's taken me forever and a day just to figure out how one frame works. If I switch to a completely different racquet in the middle of a match, I feel like it would throw off my game completely.