I think Chuck that the pitot can accumulate some ice - enough to affect the reading - long before any significant amount appears on the lift surfaces. I have seen ASI effects when there was no visible ice on the wing leading edges, and in apparent VMC.
Big Pistons Forever - I get your drift and don't disagree, but while it is trivial to establish the speed/power/pitch relationship in level flight, it is less obvious when e.g. descending especially if descending steeply, and with various flap settings, and turbulence. Sure one can do it, by applying heavy margins, but I would still not knowingly deprive myself of the ASI. It remains a vital instrument, and more so if one is going to do a max performance landing on a runway which is anywhere near close to the lower limit.