I really like this idea. In fact, I kind of wish this was the effect of Slowing Hex.
Why would I buy this over knockdown or slowing hex?
The physics of the game engine (either Source or based on it) make it so that things like velocity and friction play major roles in how you move, what you're able to do while moving, and where you can efficiently move.
For example, since you need 10m/s to slide, Slowing Hex will likely put you out of that threshold in the early-mid game. However, even with Slowing Hex, you can utilize the velocity gained by sliding down a ramp to go above that threshold once again, making it so that you can slide > jump > slide. Other examples would be gaining velocity from dash-jumps, abilities, the zipline and so forth.
If you have an item that increases how much friction affects the hero, you can deny them many more movement options. Super jumps become much less effective, sliding down slopes is less effective, and so forth. Essentially, it's a "truer" slow since it slows all momentum, not just running momentum.