I'm going to address only the last part: what is the value of having stupid people vote.

The value in allowing stupid people to vote is that your opposition can't change the definition of "stupid" to include only you and your friends.

The law is a blunt instrument. If we enshrine in law the power for Congress to deprive people of their vote, we have agreed that depriving people "for the right reasons" is justification for doing so, and now the question simply becomes "can you convince people this is a good reason to deprive people of their ability to vote".

We already have this with Felons. They are a miniscule percent of the population though, so it cannot be abused to win an election. It's bad enough the precedent kind of already exists. Expanding it would be self destructive

EDIT: Because it's been brought up, felons who cannot vote make up 1.1% of the US population.