Mohamed,
I accept (not too happily) the highway=bridge construct, since the main purpose of the bridge is to carry the highway. But the purpose of the dam is not to carry the path :)
It probably depends on the granularity in which you are thinking. Some people approach OSM with the mindset of making a technical drawing and would love to enter all the details of the dam (ideally in three dimensions). My aspirations are much simpler and I think that sometimes it's not only ok but necessary to stylize in order to keep things simple and maintainable.
So is it the dam's purpose to carry the path? Of course not. ;-) But dam and path are physically and logically closely related. Like on a bridge the path runs on top of the dam, pretty much on the line where the dam and the water meet and without the dam there would be no path. Therefore I would even go a step further and make the way of the dam and path part of the multipolygon of the lake, to which it is also strongly related.
Thorsten (Shernott)