In general, I agree with the many comments to the effect of, if what you are currently using works well for you, you should not necessarily change just for the sake of changing to a BP/W. I own a jacket BCD, several 'soft' back-inflate BCDs and several 'hard' backplates. All of them 'work'. I have found that I prefer using a 'hard' BP/W for a number of reasons that are relevant to me.
Is it my imagination that as recreational divers become more experienced they move to a bp/w? If so, why?
Some do, but many don't. Among those that make the switch: a) some do because of the discussions here on SB; b) as divers gain experience, many become more aware of, and sensitive to, their trim in addition to their buoyancy, and some of these find that the use of an AL or SS BP allows them to shift weight toward their upper body and that improves their trim; c) some others like the modularity of a BP/W system, while some appreciate the simplicty; d) some are gear junkies.
Of course many don't. Among those that don't make the switch: a) many / most never hear of a BP/W; b) some may hear of it, but in a manner that misrepresents it as something only for 'tech' divers'; c) some may hear of BP/W systems but don't have access to a unit to try out; d) some may 'try it once' and conclude it doesn't work.
How do you determine if you should go to a bp/w or stay with a jacket BCD?
Since you are asking the question, you can make the determination by trying a BP/W rig,
several times, diving with someone who knows how to wear them / fit them / use them. If you find diving is 'easier' / simpler / more comfortable, that you trim is improved, that your weight requirements drops - and any / all of these things are important or valuable to
you, then it may be time to switch. If, after a fair assessment, you see no difference, or no value, stay with what you have, which already works.
Just because what you use 'works' doesn't necessarily mean it is optimal. Just because a lot of people on SB praise something doesn't mean it is optimal. I recently bought (yet) another rubber weight belt - I already had two. I did so because of some comments here on SB about a particular type of buckle (Marseilles). It may not be optimal for me, but I want to know. And, finally, just because something is 'optimal' doesn't mean that other things are 'dysfunctional' or unacceptable.