Well, I do it because I shoot with both a 50D and a 5D Mark II. But also to give a frame of reference as to why I was making what might look like an out-of-left-field "substitute" for the EF 24-105 f/4L IS USM.
Remember, the 24-105 was designed in the days of film, when everybody shot "full-frame." The 24-105 is designed to be a pretty-wide-to short-telephoto lens. On a crop body, however, the 24-105 becomes a just-a-smidge-wider-than-normal-to-not-so-short-telephoto, and no longer fulfills exactly the same function. If you look at a lot of the EF-S lenses (with the exception of superzooms), they're typically attempting to replicate the function of an EF lens on full-frame. 18-55 (x1.6) is much like the old 28-80 kit lens. The 10-22 is much like the 17-40 and 16-35 L lenses. The 60 Macro -> 100 Macro. The 17-55/2.8 is kinda/sorta a crop version of the 24-70/2.8L. When the 15-85 was introduced, many said, "at last, we have a 24-105 for crop." And 55-250 IS is trying to be a consumer-grade crop version of the 100-400 IS (not that it succeeds and not that you wouldn't want to get a 100-400 instead).
As I said, I purchased a 24-105L and used it on crop and was happy with it. But I do find it behaves substantially differently and as a consequence is much more enjoyable on my full-frame because of how wide the wide end really goes. The 15-85 accomplishes that same width on a crop body--the one thing the 24-105 on a crop cannot do.