I am in the same type situation here.
I have both the 17-55 f/2.8 IS and the 70-200 f/4 IS for my 40D. I think they are a great combo but, I WANT the 24-105 f/4 and realize that there's a bit of overlap here. Overlap to the tune of $1,000!
I WANT the extra reach that the 105mm will add over the 55mm but don't want to be hampered by the 24mm at the other end.
I have all 3 zooms: 17-55, 24-105, 70-200, so I'm covered with large "overlaps" from 17-200mm. With my XTi, I don't really "need" all three but it sure is flexible! As you said, it's a $1000 cost for the flexibility!
I sort of fell into having all 3 by accident, but will hang on to the 3 until I decide if I go totally FF. If I stay 1.6 crop, I'll sell the 24-205. If I go FF, I'll sell the EF-S 17-55 and buy a EF 17-40. If I keep both FF and 1.6 crop, I'll keep all 3 zooms and maybe add a 17-40 for the FF (the most expensive option)..