So, I tried out a Canon full-frame camera. Wow, it had ISO settings up to 25.000! Keeeewl.
So I shot some hand-held test shot with a fast 85mm lens. Most around 1/50th second.
And what do you know? They were shaken! Shaken just enough to not be useable.
My Olympus ILC (Interchangeable Lens Camera) has "IBIS", meaning in-body-image-stabilization. So whatever lens you put on it, you get really solid help with holding the camera still, when hand-held. And I've gotten used to that with most lenses, I can get sharp pictures at 1/8 to 1/15th second, sometimes even longer. It was a small shock to be suddenly cast back to the Bad Old Days without IBIS.
So in a nutshell: Though the big and heavy full frame camera had ISO up to 25k (though rather grainy), which gained me about three stops of use in low light, the lack of any stabilization in either lens or body cost me three stops of the same. So net effect: zero.
Sony now has full-frame cameras (the A7 series) with IBIS, and the newest Panasonic Micro Four Thirds camera also has, finally. This is great. Well done those makers.
Left behind are the Big Boys, "CaNikon", Canon and Nikon. For whatever conservative reason of their own they have chosen only have stabilization in the lens, in a minority of lenses. And in many situations this costs your three stops of light. When we think of how much we have to pay (in money and weight/size) to get a lens of a given focal length which is one or two steps faster (a lot), this is just... a ridiculous waste.
By the by, related subject: I've seen it claimed that the heavy cameras make it easier to shoot long shutter times without shake. My experience is quite opposite. I think because the heavier the camera, the more the muscles strain, and there for shake. With my first M4/3 camera, I found to my delight that even without any stabilization, but with a more relaxed position, camera at waist height, screen tilted, with less weight, a softer shutter, and no big Mirror Smash (mirrorless cameras, remember?), with a 40mm-e lens I could actually take dead-sharp pictures at 1/15 second a lot of the time.