It's pretty much all been said...
For my use, the SBR's I have reduce the weight (since I'm required to have a light and a sling) and it allows them to be maneuverable inside of a vehicle, of necessary, inside tight spaces or small homes/buildings, etc. the loss in velocity isn't a hot against the platform (10.3" or my 11.5"-ers) because for these rifles I won't be engaging even out to 50yrds in real use...for classes or courses, sure, they'll reach out and touch paper or steel but the lack of velocity on those doesn't matter...inside of 50yrds it still has plenty of oomph.
I also want to suppress my rifles and my suppressed, fully kitted rifles weighs about the same as a stock 16" AR (keeping in mind I at least need the light and sling but this would exclude a suppressor).