Well, I'll say that I have a tiny little 28mm (which is what the 21mm would be on APS-H) prime that I carry with me, and that coupled with my Sigmalux (50mm) make a very solid walkaround combo. Of course, I rarely find myself wanting to go wider than 28mm, so YMMV. I think that 28mm is a very good "general wide" FL - you can fit a lot into the frame but you don't have nearly as many composition issues as you would with an ultrawide.
So as someone who does more or less what you're thinking of doing in terms of FL, I can say that it's a fine solution. I went to Paris for a weekend about a month ago, and opted for the Sigmalux/28mm combo over my 24-105L. I took about 200 shots, I'd say 120 or so with the 50mm and 80 or so with the 28mm. So they both got plenty of work.
Actually, here are two terrible shots I took for basically this exact purpose (comparing the FLs). Fairly substantial difference if you ask me. 28mm is first, 50mm is second (duh!)
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