Pentax 6x7 is exactly like a 35mm camera in every way except the size. Why bother with 645 negatives when you can do 6x7?
Because:
1. You can hand hold 645s at significantly slower shutter speeds given the same angle of view.
2. Across the board, 645s have a better variety of faster lenses. Faster shutter speeds, better hand holdability, and ability to shoot in lower light. F/2.8 long lenses is a huge plus of a 645 system.
3. Many 645 SLRs have interchangeable backs. Those 6x7 SLRs that have interchangeable backs are a good deal larger and heavier than 645s.
4. You get 1.5x more shots per roll (1.6x more with a Contax or Fuji).
5. 6x7 image quality is overkill for much work that lends itself to being shot hand held.
6. 645s are less awkward to handle. They are lighter, smaller, and much more discreet than a Mamiya Press, RB/RZ, or a Pentax 6x7, etc.
7. 645s are cheaper
8. 6x7 SLR cameras are all significantly larger and WAY heavier than the TLR that the OP already thinks is "awkward".
9. 645 cameras have an aspect ratio of 1.33:1, while 6x7 cameras have an aspect ratio of 1.25:1. (35mm is 1.5:1.) Many feel that the 1.33:1 is a more active aspect ratio than 1.25:1, and may find it more suited to capturing things in motion; a much more likely subject matter when shooting hand held.
10. There are many leaf shutter lenses available for 645.
11. Focal plane shutter 645s generally have a higher flash synch speed than focal plane shutter 6x7s. You cannot likely get a sharp hand held daytime shot using flash with '30 as a maximum synch speed. '60 on the 645 is doable with 55mm and wider lenses, and not too hard with an 80mm (normal).
12. For a given angle of view, 645s give you more depth of field, which can be a huge benefit in hand held shooting.
13. 645s are significantly easier to focus, especially in low light.
Most of these points will apply to any comparison between a larger and a smaller format.
The main issues for me when shooting hand held would be camera shake and maximum apertures of lenses.