I would use it rarely but I would use it. A good sandbox mode is super versatile and there'd be a lot of reasons to dick around in it - learn a new champ, practice last hitting / jungling, try out weird item builds and champion interactions. The first couple could be used by practically anyone once in a while, other reasons you start to get into niche / high level stuff.
They have some form of sandbox internally already, so it SHOULDN'T even be a long-term project to get something into the wild. But with their god awful project management and dev process who knows.