Is the Canon 5D MKIII a worthwhile complement to an XF100?

I said yes for the first part of your question about the quality compared to the C100 and what kind of better image it can give it to you, as it offer new opportunities and possibilities in your work.

But it all depends on your budget, technical partners on shooting and final workflow. DSLR is a great option but it requires accessories and it's not exactly set up to work "out of the box" for shooting as a camcorder.

The main difference between DSLR and a camcorder is in IN/OUT capabilities, codecs and color subsampling, pro timecode, visor, slow-motion, external recorder output option etc. You need to put all that details on the table.

I'm not really involved in pro camera but I'm sure that now they are some full frame camcorder with equivalent quality and in the same price range of 5DMark, like the Sony F100/F700, Blackmagic camera etc.

I just check online price of the body-only Sony FS100 and it cost ±4000$ - 5DMarkIII ±3500$. If you put all accessories needed, it's not necessarily in the favor of the 5D as it was some years ago, even it has it's advantage as a true camera - FS100 is only a camcorder and for example if you plan to do a documentary in some dangerous place, a 5D body is more practical in use.

In my opinion, if you work for art-school films, independant documentary or penniless fiction (as me ), DSLR may be a good choice. But for institutional work (advertising for company, marriage, promotion for public institution), I think that a camcorder is better. But with talents and experience you can do proper work in such domain with a 5D, a GoPro or a 16mm Bolex.

I run the same setup, except that I have the older 5dmkii to complement the XF100.

I only use the 5d for situations where I have time to setup and have control over the scene. For all others I use the xf100. I would feel very uncomfortable shooting any event, such as sport, wedding etc. with a DSLR, as you don't always have full control over the scene; people move around, focus changes all the time, and you just don't have the time to readjust a dslr all the time.

Horses for courses, I suppose. I am actually considering a second xf100 for multi cam shots. Will always carry the 5d with me, have a wide range of lenses and in certain situations (such as extreme wide angle shots, low light, or scenes which warrant a very shallow depth of field), I will gravitate to the 5d.

But the workhorse will always be the XF100.

The 5D has worse image quality than the C100 by a lot but it's insignificant for most clients. It's a pain in the ass to use, though. It's very "cinematic." Image quality is good enough on all these cameras.

Consider a used F3… it has HDSDI and is a real "pro" camera, unlike the C100 and 5D Mark III (both of which I own, btw).

If your question is whether the 5D Mark III is good enough for corporate video from an image quality perspective the answer is it's easily good enough. But the footage is a lot fuzzier than a real video camera. Definitely not a dealbreaker for corporate. But the lack of HDSDI and real "video" features might be.