The problem (like with voter ID laws in the US) is that it's a very slippery slope to voter suppression, and in the US we have a very creative history when it comes to voter suppression. You'd have poll workers who would present incredibly hard passages to read to voters based on a personal judgement call (read: black voters).

I (not OP) agree that dumb people voting is a problem but the alternative is to have arbitrary suppression of votes, which IMO is worse.