People who has no business voting voting. Imagine how many people would vote - not from the informed voter perspective. They'll most likely vote for the candidate they remember in a positive light. Imagine how much more sway would a big ad campaign have.
I would be ok if a blank ballot would be the least effort option with no strings attached, but with a "wasting your vote" being a stigma and a part of the culture I see no need in any mandate. I just really don't want people who can't be bothered to go to a polling station on a holiday specifically made for voting to caste their vote to vote at all. Some times gatekeeping certain things is ok. There must be a balance.