everytime i buy a lens i am buying another image stabalising system and so a degree of duplication as i can't obvioulsy use 2 lenses at the same time.
Same idea with Autofocus, Nikon (and Pentax iirc?) went the road of AF motor in the body, and drive with a screw. Then they gave up and put the AF motor in the lens (after they realised it's faster), and stopped putting the motor in the cheaper bodies, so it's a crapshoot buying nikon lenses and bodies unless you research well first.
But that made sense, in a way, lower-end-camera buyers only buy 1 lens and 1 body, cheaper to put it in the lens. Pros who keep their glass for 20+ years will want to AF with their pro body, so they still get an AF motor in-body.
As for IS, pretty much the only plus is that you can IS with any old lens up to 100 years old that you can adapt to your body. And for someone who buys a lot of glass vs bodies, buying non-IS glass can make a decent cost-saving.
Everything else is a downside though, worse performance because it's not tuned to the size/weight/focal length, overheating easier, and not active in the viewfinder annoys me no end for long slow handheld lenses.
I do hope that Tamron Idea
gets up and running though, if it's got the IQ and performance to justify the cost, then you could use any non-IS lens, on any DSLR, and you get the best of both worlds (the only downside left would be that it wouldn't be tuned for individual lenses, so not as good as in-lens, but probably better than in-body)