That line won't make it any more "provably" fair than it is now: all seeds can potentially be manipulated internally by the server (or the dice outcome can be totally indipendent from any seed for that matter). You can never know for sure what code is running internally, there's always trust involved to some extent.
Not really. That line will make it provably fair if it's on the client, and you can prove if the roll was not determined using your client seed + server seed. Hence the provable prat.
On the client it'll be exploitable by the user (hard to do but the risk is there).
Huh? You can change your client seed now.
The client cannot exploit it, he does not know the server seed (only the hash of it).
See this:
https://bitcointalksearch.org/topic/a-guide-to-how-provably-fair-works-161236Provably fair means:
1. Player can directly influence the results, but not know the end result.
2. Casino can directly influence the results, but not know the end result.
3. Player can prove the casino did not influence the result AFTER knowing the end result
4. Casino can prove the player did not influence the result AFTER knowing the end result
Currently, 3 is not true for PrimeDice.com as PrimeDice knows what each roll would be assuming default client seed. Making the default client seed generated by the client (rather than the server) would make it provably fair.
Except 1 time but I lost little and recovered soon). 
