But why is that valid, if it's trading below par? In the extreme - obviously you can't buy a call option and say 'I intend to hold this until it's $10 in the m

But why is that valid, if it's trading below par? In the extreme - obviously you can't buy a call option and say 'I intend to hold this until it's $10 in the money, so it's actually worth (time-adjusted) $10'. What's the difference, besides probabilities of outcomes?