No one. No one is seriously looking at the Russell 3000 and comparing it to the Wilshire 5000.
To the child comment:
I'm talking about two total market indices, apples to apples, and you're talking about apples to oranges. NASDAQ is a stock exchange, not an index.
Further still, even if you were talking about the NASDAQ composite, they're two entirely different indices. They do not have the same goals. There might be some relative meaning there.
There isn't much meaning between comparing two or more indices that have the same goals and constituents. You're only tracking the differences between constituents at that point, and you wouldn't need beta to do that. You could just calculate the difference between the constituents that are not a part of the total set.