Why should autofeather only be used for take off on the Dash-8 Q400?
It's because the critical case that autofeather caters to is engine failure right after V1, on takeoff, where the normal identify/feather drill uses up precious time.  An engine failure on a missed approach or go-around starts well above the low energy state of an engine failure after V1 and there is much more time and energy margin to do the normal feathering procedure.

The system is quite sensitive and there are risks associated with having the autofeather system armed and "ready to fire" you might say.  Mainly, having it put a prop in feather when you don't want it to.  Or even BOTH props in feather.  That risk outweighs the benefit of having it armed outside of the take-off case.

It's a bit like having a policy of taking a firearm off SAFE only when there is a critical need to able to fire instantly when clearing a house.