Another funny little tidbit is that, to the best of anyone's knowledge, Dwayne MacDuffie is the first person to expound the theory, in one of the articles I link to.
Which is funny, because MacDuffie is a big-time comics and animation writer, working on shows like Justice League and writing the Fantastic Four.
He relays the notion that, if character crossover equals a shared universe as a counter example, arguing for LESS rigid comic continuity.
The fact that so many folks love the Tommy Westphall hypothesis shows why MacDuffie is in the minority.
Humans love connections.
We see stars in the sky and play connect the dots. We see fuzzy lines on Mars and decide they're canals bringing water to a dying world from its poles.
This is why character crossovers in comics have always been a huge draw. People like it better when Batman can visit Superman, than they do if they're each in their own pocket dimension.