According to the wiki, a route relation with the subtype route=piste may contain points, ways, and areas .
I understand the rationale for this approach. A ski piste may consist of e.g. a narrow initial section (way), a wide central section (area), and a final narrow section (way). A relation would combine all three sections into a single entity.
However, I feel that allowing area elements in a route relation is deeply flawed. Firstly, there was a similar issue for rivers. Initially, area elements (with role=riverbank) were allowed in waterway relations. After some debate, though, this approach has been deprecated, and now the wiki discourages the use of riverbank areas in waterway relations . I believe that the same principle should be adopted for pistes as well.
Secondly, at least for pistes with piste:type=downhill (i.e. alpine skiing), dowhill direction is important. For ways, downhill direction is simply the way direction. For areas, though, this is not clear at all. I did read everything on  and other places, but didn’t find a clear indication on how dowhill direction should be implied when considering areas.
To address this issue, some people use both an area and a way, similar to what is done with rivers . I believe this is the right approach–in fact it is the only possible approach. It is also useful for routing applications. It seems completely unreasonable to add both the way element and the area element in the piste relation, though.
To sum up, in my view piste relations should never include area elements. They should consist only of way and point elements. If you agree with this idea, I would change the wiki  accordingly. I would also clarify in  the use of areas, following the approach defined for rivers.
Please give me some feedback.