I think you get the slide when you reach 500 steem power, at least that's when I got it.
To be honest up to about 500 sp, it's not like your vote is worth anything so I was never worried about it, I just upvoted anything I liked (symbolically) without caring much about my VP. It was always at around 10-20%... If I really liked something, I would just threw a Tip! to show my appreciation ;)
I think it's helpful to have the slider for new users. Seeing a lot of votes on a comment matters at least as much as it does on Reddit, where that is the main means of sorting content quality.
" I would just threw a Tip! to show my appreciation ;)"
Speaking of, thank you!