I think you're on the right track. I have the 21/40/70 combo, and the 21 was the last addition (only a month or 2 ago). I've used the 21 a few times now for cityscapes and although it does a great job, I have thought that the 15mm would have been better for that purpose. However, for shooting people I've found the 21 absolutely fantastic and I've never wished I had the 15 for that purpose.
My opinion for what it's worth is that although the 15 can shot people, it's best at landscapes, and while the 21 can do landscapes, it's best for people (obviously I'm not talking head and shoulder portraits here though).
I too will get the 15 soon enough, but for now I'm very very happy with my 21 as I mainly take pictures of people.
Here's some DA21 pics for the hell of it.
Architecture:
Cityscape:
People: