For me, I do 5gal full boils in my kitchen. I use a 8gal kettle and I like big beers so a $40 cooler and $20 of parts to build a mash tun was cheeper then buying a bigger kettle. Theres no way I can get 15+ lbs of grain in my kettle to BIAB, and thats why i use a 3 vessle system. I have a small kitchen in my apt, but using a cooler as a mash tun works great for me. Mash, open valve, fly sparge and collect my wort and im done.
Plus, I like the fact that when my grain is in my mash tun I can walk away for an hour and not stand over the stove for longer then I have to. Then sparge and again walk away for an hour. There is very little work in a mash/fly sparge sutup. I mean like none! The coolers do all the work.