Has anyone else noticed that BIAB really isn't a discrete thing anymore?...
Sure there's diversity in BIAB, just like there's diversity in any type of endeavor, product, life form, etc. But your point that BIAB isn't a true group
(a distinct style or method I assume you mean) is incorrect. Single vessel BIAB was, and still is, a distinct divergence from its predecessors. Just because it has developed diversity does not negate it being a distinct style.
...But efficiency takes a hit....
Only if you do it wrong. With a fine crush BIAB can exceed the efficiency of other methods. I consistently get 82-83% BH efficiency using one pot and a bag with no recirculation, no sparging, and no squeezing.
...stick the bag in a mash tun and recirculate through a RIMS tube'. A fantastic method, and works well (I often use the method) but moves even further from the simplicity of traditional BIAB and takes away the advantage of being able to use a very fine crush. I see some BIAB setups that are so blingy they make my homemade 3v system look like a clunker (to be fair, they still don't compare with off-the-shelf 3v systems for blingyness). Again, not really in the spirit of the original BIAB...
I agree with you on this.
There are some "BIAB" systems that have added so many unnecessary complications that they do indeed diverge from the original intent of BIAB. But that's ok.
For some folks the attraction of brewing has as much to do with their enjoyment of fiddling with hardware as it does with making beer. There's nothing wrong with that -- as long as they're having fun they're doing it right.
...My personal 'dislike' with using a grain bag (and each system has 'dislike(s)') is the messiness of handling the bag.
Handling a bag is not messy, unless you want it to be.
Squeezing a bag is indeed a hot sticky mess -- but there's no
need to squeeze a bag. Just let it drain over the kettle during the entire boil. By the end of the boil it will be lightweight and cool.
To dispose of the grains, grab the bag by the top (which is not sticky), and drop it in a bucket. Drape the top of the bag around the outside of the bucket, and tighten the drawstring. Go the the woods or to a garbage bin and turn the bucket over, the bag will invert and empty itself. At that point you can rinse it off and put it away for the next use. To me, that is easier than scooping out grains from a vessel, then cleaning the vessel.