Actually I'm also head of techs of a big vfx, anim and post facilty
I'm a premiere users since Vpro 1 and I agree mjpeg support has been better in the past, even if it's hard to compare stuff, 10 years ago we were not trying to playback realtime full HD or 2K footage from a basic premiere on a PC, but it was working quite decently for sd dv and eventually sd mjpeg.
Currently, premiere pro cs5 and 5.5 do handle mjpeg avi k-5 footage natively on my PC home and work machines, but performances are not great.
I've found a mjpeg codec (not morgan, the one from accusoft) which gives impressive performances vs premiere embedded one (I can read full hd25 whith no rendering on my 4 year old single core2 quad machine) .
This lets me think that the way codec handles the avi can dramatically change the cpu usage, and the ability to playback real time on a given hardware.
I do all my personnal editing on cs5.5 on PC (win 7) and have never encountered any corruption with media encoder also :S strange
When I need editing on vacation, I do the same stuff on my macbook pro, and it works fine natively ( it's probably based on quicktime engine directly under mac), i'm only in trouble when i need to be in fullhd 25, cause the macbook old hardware struggles to play realtime.
Mjpeg is definitely the best for image quality (vs canon h264), but filesize and codec/software compatility ruins a part of it.
Just as an info, in production context, we do generate hundreds of quicktime .mov for internal dailies of visual effetcs or animation shots, and the optimal codec so artists can play frame by frame with no pain, directly from the network is....mjpeg
Believe it or not, but AVID, can't playback those mjpeg quicktime files....