I would agree that the 24mm would not be wide enough for me as a travel lens on a 1.6x camera. But, along the same line, the slower aperture of the 15-85mm would not be fast enough for many of my travel needs when using a 7D camera. OTOH, with the superior ISO capability of full-frame cameras, the 24-105mm f4L IS might be fast enough.
The 24mm would be plenty wide enough for my shooting on a full frame camera. I use a 17-55mm f/2.8 IS lens with a 27.2mm equivalent and this is wide enough for my needs so the 24mm on a full frame camera would be O.K.. However, the 105mm focal length would no be quite long enough for my style of shooting.
So when push comes to shove, there is no one lens that I could shoot with which would fill all of my needs for a travel lens on 1.6x and full-frame cameras. But the 24-105 on a full-frame camera might come pretty close.