>>It's useful in looking at the state of the electorate from one angle, but otherwise meaningless.

Campaigning and voting would be very different if it were a national ballot. For example: under the electoral college system, CA Republicans might as well not vote for president because CA is overwhelmingly Democrat. So if 3.5M Republicans nevertheless voted in CA, how many more would vote if they knew it actually mattered? We cannot say, which is why it's meaningless to paste results from one process into another.

As an aside, Trump won the national ballot outside CA and it was just 4 cities in CA that gave it to HRC. My question would be: is it really fairer to allow 4 West Coast cities to overturn the wishes of the more populous rest of the nation? If that's fair, then why not for Congress as well - you could save a bomb by only holding the election in those 4 cities while the rest of the nation meekly accepts the result.