An Argument for Making All Champions and Skins available in Practice Tool
I believe making all champions and their skins available for use in the practice tool would provide players with the ability to test out champions and their skins before a purchase. This may lead to increased confidence in purchases, and thus, a decrease in "buyer's remorse". Generally, people who experience buyer's remorse tend to be more reluctant to take a risk on making an additional purchase.
Personally, I am often conflicted by which purchases I wish to make and ultimately end up waiting for them to come up on the free champion rotation, then try them out first on the practice tool in order to familiarize myself with their kits. I also feel I cannot get a good feel for what a skin plays like entirely by watching videos. Making these available in the practice tool should effectively resolve this for myself, and I imagine others who feel the same.
The free champion rotation would still have a place for experimenting with those champions in live combat with other players. Perhaps offering a temporary random skin option for the champions in the free rotation might also be an option.