The simply "fact" that you can hope to knock or do something to the xeno with melee is ridiculous.
Parker was a tough guy. The Alien mastered him completely in less that what it takes to notice it.
Melee, what a stupid idea
(Oh yeah vasquez in the vents, this ain't the same situation and it doesn't involve the same forces and gravity factor).
But in 1 v 1 combat encounter on ground, facing an alien with melee is just ridiculous.
Just like stunning a Predator with melee in AVP2010.
Or maybe you suggest that Arnold was a damn weak guy in Predator even with a big wood trunk?