Exactly... superior maneuverability (which jump jets grant in spades) turns terrain to the advantage of the 'mech with it.
That depends hugely on the terrain in question. If you were jumping into woods, or behind even a level 1 hill, I'd agree.
Woods are concealment, they don't stop fire, but the do make it harder to hit.
Intervening elevations are cover. They stop, partially or in full, oncoming fire at the expense of blocking similar outgoing fire.
Rough terrain, rivers, or elevation changes that are not cover are neither. They do zilch to stop or complicate the to hit roll for an incoming PPC bolt. Jumping just for the sake of distance does little if you are still standing in the open to be fired upon, which seems to be the argument here.
The Banshee here has two modes it wants to fight in. One is "In your face", toe to toe, using it's lasers, maybe a PPC if the TN's and heat aren't too bad, and two 10 point punches. But that isn't the only mode.
The Grasshopper wants to fight close, in the 2 to 3 hex range band, 2 by preference, and in this mode the Banshee wants to make the Grasshopper pay to close, generally with the PPCs. Jumping away when initiative is lost requires the Grasshopper to start closing again. Jumping away to someplace
without cover or concealment on the assumption the Banshee must chase after the Grasshopper? That is offering him a free shot before the dance starts again.
I'm doubting how much the extra mobility is effective in keeping the fight constantly in such a narrow range band, barring very terrain heavy maps the already heavily favor jump jets and close range combat.
It is extra mobility, but is it enough to be decisive is what I'm contesting. That is before you get into the Banshee's extra weight and armor being able to tank more damage.