I think since HF19, for the vast majority of users not having the vote slider discourages them from 'using up' their votes on comments.
I think having a vote slider, even with only four values (25%, 50%, 75% and 100%) would encourage comment voting. It would for me anyway.
It is a good point, yet if you had a slider many of your votes might end up being "dust" meaning they are too small to register on the blockchain.
When looking at the actual reward shares values in the blockchain, they're all huge, so I'm pretty sure even a 25% weighted vote from somebody with 20% voting power and only a few SP would register on the blockchain. I think the only way 'dust' could be an issue is if the slider allowed a weight of less than 1%, which is why I'm suggesting 25% be the minimum.