70mm is not that useful. There's a reason why you don't have a 70mm prime lens. It's close to the portrait range but not quite there.
I totally agree on the weight issue. But I'm highly skeptical of the idea that 85mm is infinitely more useful or practical than 70mm. It may be that people have unwittingly gravitated toward a convention. But I can't imagine there is anything inherently better about 50mm or 85mm, except that they are evenly spaced from other lenses in a lineup, and photographers have based their shooting around these ultimately arbitrary lengths.
85mm instead of just 70mm makes not a big difference, but enough, to me, to make sense. At 85mm you have that little bit shallower dof and that little extra reach (along with a "flatter" perspective) that can come in handy in a walkaround lens. That's actually what I miss most using my 24-70/2,8.
A 24-50mm, in my opinion, wouldn't be much popular, unless it is very fast and has an outstanding IQ. I once had a Pentax 24-50/4, but I always preferred to carry around three primes, which were much faster and had a much better IQ, since I had anyway to switch lens quite often, with such a small focal range zoom.