IMHO
I buy a gun to shoot ammo, I have a s&w m&p 40 that will shoot any ammo I put in the magazine, I have a s&w 640 that will do the same, Shotguns likewise, and I have a AR 15 that will, like the before mentioned guns, shoot ANYTHING I feed it. I have had guns in the past (note the use of the word "past") that would not shoot certain kinds of ammo, guess what.....I SOLD THEM.
When I go to the store I buy ammo to hunt with or to shoot at targets with and I usually buy ammo that is on sale or whatever is the cheapest unless I am using it for a certain purpose (self defense, different animal hunting, etc). I don't want to have to walk into Walmart or wherever ready to have myself a "range day" and say to myself "Well they don't have brand Joe Blow awesome copper lined, steel cased jacketed hollow point #9. Since that's the only round my gun shoots I guess I have wasted a day since they don't have the ONLY brand of ammo my gun will shoot and I can't go to the range"
A reliable gun shoots any ammo IMHO.
Maybe I am over thinking it but the way I view my gun purchasing and collecting is like this, If I were to ever be in a situation where I run out of ammo for whatever reason, I should be able to pick up ANY spare ammo laying around, of the same caliber as the gun I am carrying, and load it into my magazine and use it effectively. If I cannot rely on my gun to do this then I have no need for it.......but that is my opinion.