The Drunken Lizard Pub

As far as my experience shows, enemies detect your character on two separate levels.

First, they check to see if they can see you - that's what the hiding is for. If they can see you, they start attacking you.

If they can't see you, they check to see if they can hear you - and that's what moving ...

I'm playing hunter/rogue. Have 3 points in move silently and 7+3 in hide in shadow. My question is: Does move silently anyhow is usable? Even i'm not silent i'm hidden in shadow and therefore can't be hit. Why should I invest points in move silently?