You don't lose the flexibility of having multiple weapon specs when you take HPM. You still have two specs, and can use their flexibility to your heart's delight. You just lose one legacy slot. What you get is the ability to boost your melee offense/defense in situations where spec skills aren't needed. The most obvious example is mob fighting, but I'm guessing there are also some pk situations where "super-melee" is worth giving up spec skills. Especially snice you still get pincer-like lag via exotic skills like batter.

For instance, if you're an axe spec fighting someone whose weapon and shield you can't destroy, and you're just going to be spamming pincer, then, if you had HPM, you could achieve the same lag but have better offense and defense.