You're right. But the horary isn't going to tell you something will come to pass that isn't in the natal promise.
Predicting the future is no easy task. I suppose if you're certain you know all the events and how they'll transpire in a native's life, you'd have no need of horary at all. Then again, if you're that good, you probably have no need of astrology at all.
But I don't think you're giving horary enough credit here. Yes, you'll make mistakes. You'll make mistakes in natal interpretation, too. If you try to merge the two, or compare them before the fact, you're likely to make twice as many mistakes, not fewer. At least that's how I've always seen it work out.
Horary is a wonderful stand alone technique. There is no need to complicate it.
Mistakes and proficiency can be controlled with knowledge and experience but are not part of this dialogue: the issue is conceptual. Applying a second measure as per astrological lore (i.e., natal promise) can only strengthen the correctness of the conclusion, even if it confounds the astrologer. We must draw a distinction between proficiency and concept. If there is not proficiency and experience, as with anything, it makes sense why working with two charts one does not understand can yield double the mistakes. Excluding additional, double-checking analyses affords increased likelihood of mistakes. It does not afford greater control, as you imply. Proficiency and concept are complematary of course so, with both, the horary should reflect the natal promise in an accurate reading.
You are entitled to disagree. Before you do that, however, the burden of proof rests on your existing natal data which remain uncorrelated with your existing horary data. The claims you are making are not tested. It has been established that astrology is complex, however, that is all the more reason for continued research which can make us better practitioners and theorists as the art lives on, rather than being indocrinated misers who carry the moral burden of misleading with incorrect predictions
*. Prediction being difficult is not an excuse. Doctors could say the same thing. A responsible practitioner is dedicated to life-long learning.
* Comment is conceptual; not personal.