Rai-
For a couple of reasons. First it makes it so that it has to be actively restrained, rather than oh I want to activate this so I will. If he were to keep his eye open he would be constantly in the infant stage, until he passed out from chakra exhaustion. In addition it is used as a balancing factor. Originally this KG was going to be used with a genin. Having to keep his eye closed would make it so that his field of vision would be limited, and his depth perception would be off. Daisuke is much older, and as such has gotten used to these side effects. Besides that would do you think of him?

Everyone else-
Thank you, I am very glad to know that you all like him.