Shiai tokens are already currency lol. I understand what you mean, and there is no reason not to do this. A lot more plays into this as you think. As of now, the GMs generate shiai tokens in order to send out prizes, and then when someone wants a prize we obliterate the coins and generate more when needed.
Although this is not that big of a deal because the GMs are mostly active and can get prizes to you relatively quick, it would make it more convenient for the community to have ST items in the shop. I will support this idea on the basis that it is more convenient for the community, and it makes things a bit more simple for everyone.