I've never really felt a use to turn into the slayer form, but maybe that's just me.

Changing to a slayer is a good way to Lower Rep. points (if youre evil). And its a good way to die too. You should only change in to a slayer only If it is really needed. For Example...
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You could use the slayer change when youre in the underdark, at the point where you find the door that says "Only a strong mind or Godly strength can open this door".You could change into a slayer to open that door, or capture a mindflayer to do it for you.