It's not a key component, but it is a significant benefit of their system. The main reason though is that the primary benefit of this system is flexibility of placement. By locating grid storage close to where it's used most, you save significantly on transmission losses (and infrastructure). Finding sufficient vertical cliffs would be even more difficult than finding suitable locations for pumped hydro.

Also if you expose the weight to open air and wind the whole system would be much trickier than using an enclosed shaft.