I did exactly what you are thinking. I already had a 17-55mm EF-s which was wonderful, but when a new kit 24-105mmL came up on Craigslist for $700, I bought it. Then a 70-200mm f.4L IS, plus a 50mm f/1.8. Then, when I bought a 5D MK II, I did not need lenses immediately, and I kept my 40D and the 17-55 as a backup.
The 24-105mmL is great on a crop or FF, but on a crop, I occasionally had to change lenses, since 24mm is not always wide enough.
Later, I bought a refurb 7D with refurb 15-85 as backups, those were both excellent, but when I bought a 5D MK III, the 7D just languished. I still have the 15-85mm ef-s just in case I buy another crop camera some day. I also have a 10D that I bought for the lenses that came with it, it is just sitting as well, but its a cheap backup and does quite well.