I'd love to see what your solution is in low light for manageable depth of field. Stop down to F5.6??? I don't think so! Also there are no PL mount adapters on the 5D Mark III. Mirror in the way and sensor too large for most Super35mm lenses.
I have no problems shooting f1.4 with the 5d mark III. I really wonder about the skill of people who can't do it.
Yes, it is quite difficult with a 70-200mm but with a 50mm it's already quite manageable and looks absolutely great in lowlight.
I haven't used the FS100 but the 5dmarkIII is better in lowlight than the FS700 I used. Same shooting locations, 30mm f1.4 on the FS700 and +30dB gain = unusable. ISO 10 000 on the 5dmarkIII was usable if the shadows were slightly pushed down.
I also wonder about some of those comments about shadow noise and compression issues. Granted, I usually shoot at intermediate ISOs which tend to be quite clean. But I get absolutely great shadow noise. For example the shadow noise with the FS700 is quite bad in higher ISOs. It gets blotchy and murky. Same with the nex-5n although even worse. You can push it down with the FS700 because the dynamic range is off-the-charts but with the nex-5n you can't.
Everything depends on which you compare it to. If I would compare it to the Alexa and BMC then yeah... it's not as sharp or detailed and has more compression artifacts. But when doing an actual shortfilm or musicvideo, I feel that the positives really outweigh the negatives. When I got the 7d I was like "you know...if they fix the aliasing, moire and less compression...I would never need another camera". The 5dmkIII basically did it. I have no need for anything better, until there is a significant difference. BMC looks to be great but I can't use RAW anyway until I have lots and lots of cash to burn.
Plus stills. I've fallen in love with full-frame photography.