While the effective field of view on a crop camera may be the same as a longer lens on a 35mm, the focal length does not change and therefore neither does the aperture. A 17 - 55mm lens may give the same FOV as a 27 -88mm but it's still a 17 - 55mm and the relationship between the first element and the focal length remains unchanged as does the aperture.
True. But not really the point I am making... 55mm on a crop body is equivalent to 88mm on FF, and f/2.8 is f/2.8 regardless. But for the same framing, you'd be closer to the subject with a FF body, meaning shallower DoF. It also means the perspective is different, so it's not really the same shot. But from a technical standpoint, on FF DoF is effectively
shallower, and when comparing current FF sensors to current crop sensors, ISO performance is improved so one can increase ISO by a stop to 'make up the difference' between f/2.8 and f/4 (in terms of shutter speed) with no noise penalty.
What that means, and my main point, is that functionally
the 24-105mm f/4 on FF is approximately equivalent to the 17-55mm f/2.8 on a crop body (from a DoF and exposure standpoint, plus it has a broader focal range). So the "I'm not going FF yet because there's no FF equivalent to the 17-55mm f/2.8 IS" argument is a fallacy, since on FF, f/4 + better ISO performance is comparable to 1.6x f/2.8, and both lenses have IS.
Now, if someone is waiting for f/2.8 IS on FF because the whole point of moving to FF is to achieve better
performance, rather than comparable performance (from a technical standpoint), I can't argue with that - so, if that's the case, keep on waiting for the 24-70mm f/2.8L IS.