Originally posted by Lord Lucan I don't really care about the camera body being 26mm thinner. Maybe someone would who wants to put their camera into their pocket or handbag, but then we are talking more about the P&S camera market. And the thinner body is no advantage if you then need to put an adaptor on it to make up the thickness for those legacy lenses, or indeed if the lenses for the mirrorless body are themselves longer for optical reasons - otherwise even standard focal length lenses are required to be of reverse telephoto design (a compromise) as most SLR wide angle lenses are now That's all fine for P&S and the budget market but less than opimal for the top end. I hate adaptors anyway, just something more to fiddle with and go wrong.
Whether or not you care about the body being thinner for the sake of physical dimensions, the short registration distance gives a lot of flexibility in adapting MF and, in some cases, AF lenses from different mounts. That may not be of interest to you, but it vastly increases the number of other-system lenses that can be used with the camera. A good number of folks seem to value that... not just those that enjoy vintage glass, but those who might have favourite lenses from other systems.
If I'm shooting my A7 MkII with my A-mount lenses, I take it in my camera bag with the LA-EA4 AF adapter already fitted, and I have no need to remove it in the field. In that application, it's no smaller than a reasonably compact DSLR, but that's fine because I'm treating it like a native A-mount camera. Actually, it's still smaller and lighter than my HV (Sony A99), and whilst the camera plus adapter combination might look a little ungainly, it handles perfectly well even with bigger, longer lenses, despite the thinner body (in fact, the A7II + LA-EA4 adapter is remarkably similar in size and shape to the Pentax KP, with its thin body and protruding mirror box).
If I'm shooting other glass - which in my case is mostly vintage manual lenses in M39 RF, M39 SLR, M42, K and F-mount - I take the camera, an adapter or two, and the lenses I want to use. Again, size isn't a concern for me. Flexibility is. I don't care about being able to put the camera in my pocket or a handbag like a P&S. I do, however, care that the short registration distance allows me to use almost any lens on that one body - and it's the only camera I own that allows me to do that.
As for adapters being something else to fiddle with and go wrong, there's nothing to go wrong with a dumb adapter. For AF adapters, there's no more in those than you'd find in a DSLR or SLT camera. It's just in a separate unit that attaches like a teleconverter - and most of us are comfortable using those. Keep the contacts clean on the body and adapter (as you would a lens or TC) and you're good to go. Once it's fitted, unless you want to start using another system's lenses, you can leave it attached to the body and treat the camera plus adapter as one unit