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I don't really think so.
IMO most of the current coin is an-average coins being listed on the casino, I guess they only listed the most regular coins listed on all casino. So, If user want to moving from one casino to other casinos.
They no need to go in exchange first, since you can make a direct transaction between the casino. Remember, add a coins need a few years in "Stake" ~XD.
If you talk about casino that is not famous it is true, even they add high risk coins but this is Stake they must consider something
Casinos with a low reputation is impossible to compete with stake, therefore they prefer to attract players from the coins community
I can see stake using the same method this means stake is one step ahead of other famous casinos.