If I go with the Tamron 17-50 and the Canon 70-200 f4 L lense I will have an effective gap between 75 and 112.....and if I go with the Tamron 28-75 I will overlap on the long side but my widest will only be an effective 45mm....18mm short of the effective size of the Tamron 17-50. Also, if I ever go to full image sensor the Tamron 17-50 will be useless.
I would not be concerned about a 'gap' from 50 to 70mm. I suppose there has been an occasion sometime that I've wanted longer than 50mm and shorter than 70mm, but I can't think when.
Not having anything wider than 45mm equivalent would be constraining IMHO, and if you ever end up with a "useless" 17-50mm lens you can sell it. The millions of crop factor DSLRs aren't going to disappear even when/if full frame becomes affordable in the price range of the current 30D, and besides, that isn't going to happen this year or next!