I never shoot video with my mk2. I only take portraits, no issue with slower shutter speeds etc. Pragmatically, what would be the main disadvantages of downgrading, in terms of output and image quality?
If you shoot under 400iso and don't print large, you won't notice much difference in the image quality. If you shoot higher ISOs, the Mk II has less noise.
Practically speaking, the screen on the MK I is the most annoying thing if you're used to the MK II. Colour accuracy and sharpness during playback are poor compared to the MKII and newer cameras- and far less useful for evaluating focus.
The main benefits of the 5D MkII over the 5D MkI include:
- Addition of ISO 3200 and 6400 native ISOs
- 21MP over 13MP
- Addition of dust reduction features
- New menus and interface
- Vignetting correction built in
- AF Micro Adjustment
- 98% viewfinder instead of 96%
- Higher resolution larger LCD screen
- Live view for composition
- Twice the battery capacity/frames per battery
- Silent modes
Sure most of the above isn't obviously impacting image quality(beyond the high ISO features), but it sure will make it more comfortable and easy to get those shots.