I'd say it's simply for peace of mind. IMO most people do not research the pros and cons of buying ammunition, and that comes from my experiences at gun shops. The vast majority of people I have come across go into a shop, and eat up whatever the guy or girl behind the counter says (mainly for gun purchases). Right or wrong, that information is taken as the gospel.
And in the current market with 439 different types of SD loads, I think convienence comes in to play as well. I may only buy ammo at a particular store, and that said store may only carry Critial Defense (or insert whatever brand). If that brand shoots well, I as a consumer may not even consider anything else.