Will accepting altcoins make a difference?
Probably it will , as altcoin that arent regulated yet might be consider just as a token or like monopoly money. Although some countries specifically banned online gambling so it will not make a huge impact for some country
Is that the reason why JD is using clams because it is allowed in US? I don't understand the reason behind this suggestion.
Tried to dug for old post and I found this post describing what happened to JustDice and the reason of it accepting clams
https://bitcointalk.org/index.php?topic=238613.msg9853803#msg9853803, I think this posts sum up on what happened at JustDice
Couldnt find anymore relevant post regarding this , perhaps you can ask dooglus personally about this