Okay so its a force axe that makes the sorcerer worse at melee combat... but whats the up side?

Besides looking freakishly awesome

https://www.eternalcrusade.com/rogue-trader/217-maelstrom-axe

It says worse at defense, which implies it will have more damage.

well, it can easily be interpreted as meaning...

less effective for blocking and defense THEN A NORMAL POWER AXE (equivilant)

EDIT

and besides, so far we have no information as to how the "force weapons" will work,

if they use a warp charge to inflict far greater damage with a power type attack or whatever, so until we know more I wouldnt go worrying too much lol

Personally I'm hoping using a force axe on an enemy powers up a sorcs warp abilities. Although really sorcerers are dirty and should have their skulls cleaved. Blood! Blood! Blood!

I keep my robe of Khornite skins quite clean, thank you.

lol, ya it is odd they dont tell you what the upside is other than being METAL of course

Maybe that is the upside, upon gazing on the axe the enemy involuntarily starts headbanging to the beat of a brutal tune, leaving himself open to a decapitation.

I have to wonder if they don't give a solid mention of the advantage of the axe because they haven't exactly nailed down how exactly force weapons in general will work.

Spot on.