G'Day,
For me extra leagues would be no better than practicing, in fact probably worse as you cannot focus on a single area of your game in a league. Like ten pin spares.
As a kid watching Terry, Lovey and dad, these guys would hit their spot a lot. For Dad he bowled Friday night singles and 3 games extra in practice if a tournament was coming up.
I think some people can hit their spot a lot and others can't no matter how much they play. Me, sometimes I can hit the side of the barn and others I can't. The main benefit with practice for me is my spare shooting gets better. My strike rate is the same. But hey, I am just a League Hack.