Unless you think convenience and stability are reasons...
My ScubaPro Hydros Pro is more convenient and more stable than any BP/W I've tried - including the Freedom Contour back plate that I own. Actually, the Hydros and FC are probably about equal on stability. Neither one moves on me, like a traditional BP can. But, the Hydros definitely wins on convenience (of fitting, donning/doffing, and packing).
For single tank diving, the real benefit of a BP/W is cost. If I didn't get my Hydros at shop employee pricing, I would never have bought one and I would be perfectly happy diving my BP/W rigs forevermore.
Funny enough, since I bought mine, 1 of our shop's DMs and a DM Candidate have both bought Hydros BCs and one of the other instructors tried one this past weekend and he is now buying one, too. The other instructor is also a tech diver with his own assortment of BP/W and integrated BCD rigs. The Hydros is going to replace the Apeks Black Ice BCD he's been using. From what he said, it was the super fast drying time, in addition to the convenience, comfort, and stability, that really sold him on it. Bringing a still-wet BCD home from someplace like Cozumel I guess pushed him over the edge.
With the travel waist belt installed in place of the Hydros QR weight pockets, it is a very streamlined and uncluttered rig. At that point, the only real knock I would give it is having a too-large air cell. But, the bungee setup does keep that pretty well in check. And with that big air cell and the convenient size adjustments, it's a single BCD that is easy to use for warm water and switch right over to diving cold water (e.g. in a drysuit). With my Freedom plate rig, changing it from rash guard sizing to drysuit sizing is a PITA. Adjust the straps. Then adjust every D-ring to be in the right place with the new shoulder/waist strap positions...
