I have to confess I used my 12-24 too much and my two 28mm are rarely used
I agree that sometimes even landscape would benefit from a little short telephoto treatment (as MoiVous said). So, I'm thinking to use 28mm more often for landscape because I feel my pics with 12mm (or 18mm APS), most of them are taken too wide (i.e. trying to include too many things to the frame)
But indoor, I found 12mm is very good, as a plus I could go up to 24mm if I want to. It's only F4 max and I have 28mm F2 but still I used 12-24mm more for indoor. F4 is good enough to get unshaken pics indoor most of the time.
So, back to the thread subject, 12mm (APS-C) is wide enough for me, and in fact sometimes too wide, need to use something less wide.. thinking of 21mm ltd (argh LBA strikes again :ugh