To be a bit pedantic, camcorder manufacturers were putting OIS into camcorders a good decade before anyone put video on their IS-capable DSLRs. I had a Sony TRV-9 with OIS way back in '98. I know it hit still camera lenses a few years earlier, but it didn't take very long at all for it to trickle down to video use.
As a result, I suspect that at least a hundred people use some form of IS for video (on camcorders) for every one person who uses it for stills. So yeah, it's mostly used for video—which is not to say that it is only useful for video, just that statistically, it is mostly used for video.
They are different systems. For a start the video image device on your TRV9 is probably 1/4", and the lens will be tiny as well.
The system developed over the years and on my last compact camcorder (Sony Z1) you could specify how smoothly the IS operated. This is a feature that DSLR lenses just don't have. My SX230 lets me choose whether IS is on all the time, or just for the shot, maybe this is what we need on EOS bodies, if the mount protocol exists (which I doubt, as the switchgear is on the lens)
Also, I don't agree that serious video requires a support system. Sometimes it does, but not always. I've shot a fair amount of serious video without a tripod. Sure, you can't survive shooting for hours at a time that way, but if you're just trying to grab a few seconds of B-roll, you can get it a heck of a lot faster if you aren't having to drag a tripod around, in my experience. It's a compromise, yes, but often it's the right compromise.
A trv9 is a different prospect, ergonomically, size, distibution of weight, to a DSLR. My old TRV-900 could be hand held with OS on, but it was desgined to be held that way, unbraced. DSLR's really are designed to be braced against the eye... not the way folk use them in video mode.
I've got various grips, from a simple L bracket, to a shoulder rig to full ENG tripod. And I always have a superclamp and microball head in my kitbag. I love tripods for wide establishers and interviews. More often than not my camera is on my shoulder, or a monopod.
The form factor of a DSLR is just wrong when held away from the 'brace against face' position. So IS isn't a bad thing, just so far Canon haven't got it right for video, in their EF lenses at least (the XL lenses had a nice implementation, that said)
IMO, some form of IS is critical for long still exposures and for video for very different reasons. For long exposures, it is needed because otherwise you get double images. For video, it is needed because otherwise you get seasick. Either way, it isn't always useful or necessary, but when it is, it's a godsend. Just my $0.02.
For long exposures you really want a tripod. There are limits.
I look forward to seeing what the proposed nifty fifty 2 offers. If it's a well implemented contiguous IS with a smooth mode that doesn't cost the earth I might be interested.