A little question (well not so little)

[Makrontt le nécron]

I was playing again HL2 recently an then I realised that the weapon movement is not the same in HL2 and its episode and in HL2 DM (and OC too). Weapon's view models don't stay rigged to the same point of the screen, I mean if you look up, the weapon is a bit up, if you fastly turn to the left, the weapon will be more on the right.
Why it's not working this way multiplayer? Is that performances issues? Client exploits?
If I ask, that's because it's a nice feature and I would like to see weapons that way in OC (is that possible?)
(so yea it's a bit a request)