Why would you stop at 21 instead of 24?
I can't speak for the others, but I went 21 on my rogue in early MoP due to the scaling in the 20-24 bracket. 21s were better than 20s because of dual BvB, and I just never felt like leveling it to 29.
YesDoes a lvl21 BvB scale better than a lvl24 heirloom dagger? I suppose as a rogue you aren't giving up any useful abilities not getting to 24.