Perhaps we need to make a distinction between two types of disability

1. Physical disability.

This is like, they're in a wheelchair, because their legs don't work. Because of an injury, or a long-term illness.

Neither of these afflictions should disqualify them from voting.  They don't stop their minds functioning. Their minds are quite ok. They can make rational decisions.  Such as who to vote for, in an an election.  I would accept any vote from such a person,  as fully valid.

However, this is different  from a person who has - 

2. Mental disability

This is like,  their brain isn't working properly. It's not rational.  From whatever cause, they can't think properly.

So why should we take account of a vote, from such a person.  They're clearly not competent.

This argument seems fairly reasonable.  What's wrong with it?