These are the five most common advantages that I often hear cited:
1.
A jacket keeps you vertical on the surface.
I can honestly say I have never had a problem staying vertical with a BP&W. If it is pushing you forward, let a bit of gas out.
2.
A jacket has pockets.
You can buy pockets that fit on a one piece harness, use drysuit pockets, or wear cargo shorts over your wetsuit. All the kit I need on a single tank 'recreational' dive can be clipped off on the harness in a neat fashion. Also, how much crap do you really need to take in the water with you?
3.
A jacket is easily adjustable.
How often do you need to adjust the harness? Once it is set up, you shouldn't need to touch it very often. For dive schools / hired kit, there is an obvious advantage as it will fit any customer, however, you can buy 'comfort harnesses' for a BP&W system or use something like the
Halcyon Cinch system.
4.
You can get integrated weight systems on jackets.
You can have them on a BP&W set-up too.
5.
A jacket is more comfortable as it has padding.
Padding that fits to a BP&W is available too, or you can opt for the comfort harness. The truth is however, that the padding doesn't make a blind bit of difference. If anything, a simple one piece harness is more comfortable as there is less material restricting your movement.
A BP&W can be used for most types of diving and is easy to reconfigure, for example if you want to dive with twins. When the harness eventually wears out, just buy some replacement webbing. The backplate is pretty much indestructible and if the wing fails, you can easily replace it and keep the harness you know and love.
I use my BP&W exclusively for easy single tank diving these days; any deep stuff and I take the rebreather, which has a built in harness and wing. I would not go back to a jacket for any reason.
The only valid advantage of a jacket is item #3 when applied to hired kit. It is what holiday divers know because it is what they have usually been taught on. If dive schools worldwide started teaching with a BP&W from the word 'go', jackets would be redundant.