Nicking things seems like it will inevitably lead to them being 'enhanced'. If paying for Premium classes of travel, I really don't fancy eating my meal with a plastic knife and fork or drinking out a disposable plastic cup - really would cheapen the experience.

Naturally if they are made available to you (e.g. in the same way hotel toiletries are) then it's fair game.

I'd never dream of taking a glass off an aircraft. I will however admit to nicking a pint glass from a pub in my younger days; an act from which comeuppance was dealt swiftly as I dropped and smashed it after stumbling on a broken bit of pavement. Lesson learned.