As to what people will find more streamlined and easy to vent will depend on their own experience. My experience has been that every BP/W has been superior to every vest BC that I have tried in both streamlining and venting.
The streamlining part is such unscientific BS it is just hilarious. 
For a take on your venting issues, perhaps you chose badly designed Vests? Or there are some other ideas between the lines at the end.
A BC is more streamlined than another if it cuts through the water easier than the other. In order to say that one BC cuts through the water better than another you need to make measurements. Until someone publishes some verifiable measurements all we have is anecdotal opinion. When someone is just anecdotally opining it is improper to say something like "BP/W's are more streamlined." The Proper thing to say is something like "I have never measured it, but since my body now hangs free below my BC it feels like I am more streamlined" or "I bought a cheesy Vest with not much investigation and when I was a new, overweighted diver it was hard for me to dive in proper trim, but now that I am more experienced, use the proper amount of weight and am forced to horizontal by my rig, I am more streamlined."
You have no experience in BP/Ws. I would suggest you try one before commenting as to how one might experience using them. I don't think any of your claims are valid based on my own experience.
My initial response was directed at you because you
criticised the OP for so called lack of experience when in fact you have zero experience with BP/Ws.
This is a large part of why this is so entertaining; you continually type things that are completely false, in more than one way; your initial response was post #20, I did not post in this thread until post #22. Your initial response "to me" was because of my posts quoting In-The-Drink, who is not the OP, and technically what I said was in general, not speaking directly to him, but playing with the incomprehensible non-sentence he posted; "I'm pretty sure - anecdotal yes of course - that this is because I am a point to and thru the water, rather than a 45 degree large surface area."
There is no reason why only people diving dry or with doubles should be the ones to stick to BP/Ws so I am not sure why you even raised it in the first place.
There is no reason I have to type this three times in the same thread! This is not what you keep calling it! This is not me saying only dry and twins divers shoud dive BP/W! This is me saying if you are not going to dive dry or twins then you are making the BP/W decision for reasons other than the two reasons that make the most sense to me, since the vast majority of BP/W divers I know of dive dry &/or twins.
If someone is just a typical, average diver the reasons for considering a BP/W are not so pressing, IMHO.
Also it is irrelevant as to what the 'majority' of divers are doing as equipment should be based on individual comfort and need.
The majority of the divers you dive with are diving BP/W, but I'm sure that had no bearing on your use of BP/W what so ever. Right. Why in the world would you not consider the most popular gear. A smart person would look at the people like them, doing the things they like to do, where they will be doing it, the way they like or want to do it.
The OP may very well be right in his consideration of BP/W; I never questioned him considering a BP/W. I simply offered some rebuttal to streamlining as a reason and added that venting from a bladder farther above your body makes you move your body more, all other things being equal.
I personally am not impressed with the design of most modern scuba gear; I believe marketing is taking us backwards with regards to funcionality. I do not know of any well designed Vest BC's currently being made, but there are many good ones being sold cheap on eBay every day. Likewise I have not seen any modern BI BC's that I think are designed well, but The Balance I got from Salvation Army for $20 serves it's purpose; carry slate elegantly.