I know I have this behaviour with the DA10-17 fisheye, but it is not so strange since I would actually prefer to have a fixed fisheye and an extreme wide angle prime, so that is how I use it.
The DA*16-50 is too early to say, I recently got it.
My other frequently used zoom is the DA*50-135, and there I realy try to use the middle range and avoid the extreme ends. In both cases I have good primes that does better at 50mm and 135mm. When I've tested the DA*50-135 against my A/A* primes, it is clear that the DA*50-135 is not that very good at 50 and 135mm (less sharp and perhaps some light fall off, people say it also have some distorsion, but my brick wall is not regular enough for me to be able to see that
), but in the middle at 85 and 100mm it compares well against my best primes closed down and OK even at f2.8.
I believe this is the typical behaviour of most zoom lenses. They are optimised and constructed with some compromises that works better in the middle of the zoom range and not so well in the extreme ends, or else they would be impossibly heavy and expensive. Therefore it is actually quite stupid to only use the extreme ends, since you don't get the best out of your lens.
As far as I can recall, from when I was still shooting a lot with the MZ5, the FA28-70 f4 is one lens that behaves quite well over the whole zoom range, but it is probably easier to build a zoom with such short zoom range.