Seriously? Lighter, cheaper, and in the case of the 17-55, no FF equivalent (i.e., FF equivalent 27-88 f2.8 with IS).
Again, I chuckle. Guffaw, even. While it's true that there's no 2x-7/8x f/2.8 IS zoom for FF, the crop factor affects DoF for the same framing and FF sensors collect more total light meaning lower noise. So, the real
FF equivalent of the 17-55/2.8 is a 27-88mm f/4.5 lens, meaning the 24-105mm f/4L IS on FF is wider, longer, and faster (DoF for same framing), still has IS, and is a couple of hundred dollars cheaper when bought as a FF kit lens. The >1.3-stop lower noise more than compensates for the loss of 1-stop of aperture, in terms of shutter speed. All you lose are the f/2.8 AF point(s). Don't get me wrong - I loved the 17-55mm f/2.8 IS and I think it's the best general purpose zoom for APS-C, but the 24-105mm f/4L IS on FF is better spec'd and delivers better IQ than the 17-55 on APS-C.