HF is nice to have as either side, but it isn't decisive, and is not worth sticking your neck out for. If the Union player wants it enough to commit early, you probably can't stop them.
I have heard that in real life it was a less defensible location than its representation in the game: once either side can occupy it in force they can keep it fairly easily as it is difficult to enter the region from the east, north and south, and the western approach presents logistical challenges.
From HF the Union has a secure forward position to advance down the Valley, but must fight their way through several easily defended chokepoints. Assembling enough troops to force their way down the Valley risks leaving Alexandria or even DC dangerously exposed.
From HF the CSA can theoretically strike into Maryland, but this is rarely successful unless they are already winning, as the Union can easily counterattack with much larger forces, potentially trapping the CSA on the wrong side of the Potomac and destroying the army altogether. You could conceivably push into WV, but nothing is actually valuable there until you get to the Ohio river.