You are assuming 2 craft of similar manuverability. For a P-51 vs a Spit, there is no ability to outmanuver it. A 190a8 with heavy guns package against 90% of the planeset is dead meat. It's not that the HO is bad because it negates the next move (I agree in general, though) it's perfectly valid because sometimes it's the only option.
For 2 similar craft with moderate manuverability I agree setting up for the HO if the other guy sets up for the merge is a bad move. You blow E and possible position for a quick pot shot. I know when I see somebody do that to me I have the upper hand..... that is.... IF they manuver. 90% of the time a HO just runs outside icon range before it's clear to turn back.