I agree with you; known values have more value than the potential of unknown ones. As i saw in another thread, i wouldnt trade a Guhle for a defenseman available at #6 on the off chance he is better. (I'd at least need more reasons like salary, congestion in that position, etc); the potential difference in skill wouldn't be worth the risk.
Pospisil seems like a hard worker in Calgary and seems to be getting a lot of love there. I thought he was a 3rd liner, but some posts have him as a 2nd or higher. Anyways, my point would be that if you have a known value whom you see as a 3rd liner and you can get a player with potential as top 6, then yes, i woudl say the difference in skill would be worth the risk. You could probably argue that this wouldnt be a "crapshoot" though