In live sound, you will always want to try and get the backline to be as quiet as possible. Low wattage amps are perfect for that because they hit that "sweet spot" at lower volume levels than big amps do.(ps: with the 5150, there's not much of a sweet spot to hit below 120dB...)
If you have a full blown stack, and you're running it below its potential/capacity, then what's the use in dragging that monster around to every gig or even owning it? Sure, two 4X12 cabinets look cool and all, but you're only going to end up mic'ing one of those drivers!

The only reason anyone should ever really need a stack is if you're playing gigs where there is no PA system available...