Well apart from a brief period there was only one Rotary on the South Park and that was used mainly between Como and Leadville. Keeping it simple.
And that involved going up and down twice.
If there was a significant benefit, I assume going down will be much easier it was never mentioned in anything I have read. Probably because it was not relevant, they had no choice, it was what it was.
There may be other factors, for example going down through thick snow the snow acts as a brake, thin snow and you have issues keeping control and have to go more carefully.