Well, the weight...
It's a pity I already have 24 and 35 Sigmas. This lens might be heavy, but following your MTFs it's better than both Sigmas in terms of resolution, bokehwise not worse and the minimum focus distance only 3 cm longer than with the 24 mm (and 2 cm shorter than 35 mm).
So, yes, heavier.
But: it doesn't need much more space in the photo bag - in fact, 24 and 35 put together with a twin rear cap is longer, 400 grams heavier and 580 francs more expensive while both lenses, although primes, don't reach the MTF of the zoom.
Only problem - for people owning already the primes there's not much to gain. Except the advantage to have three primes in front of the camera without constantly exchanging them. It is not a discrete lens - but with a fat DSLR behind it, none of the primes are, either.
Okay, another downside might be the necessary AF adjustment if one uses often wide open aperture. Usually one can set up 16 combinations of distances and FL with the dock which is timeconsuming.
The value of the lens is massively better than the primes, not to speak of the genuine Nikon ones. If I take the Nikkor 35 and 24 f/1.4 in the shopping basket, it's the value of 4 (!) of these Sigma zooms. Which means, I could afford cracking three of them and would sil save some money with the 4th