As previously mentioned, the canal is one conceptual object and thus should have one area representing the area it takes up. It is also significantly easier when edits to one part of the canal do not generate a changeset comment the geographic size of the canal. When an area is attached to a relation of any sort it can be a pain to detach it and update it at a latter time so it is better to do it as a multipolygon from the get go. There is also the fact that when multiple land use areas share parts of polygons but are not mapped as multipoygons there is duplication in the data and that is something I try to avoid in Miami dade county, I also find it significantly easier to update and fix land use when it is mapped as multipolygons rather then simple polygons especially when mapping areas that cover relatively large amounts of space. 

As for the grass along the highway it would anyways need to be done as a multipolygon as I am mapping the barriers and other such features along the highways in Miami Dade and as I go I add the land cover along the right of way of the highway and attach the land cover to the barriers added, and as to (as mentioned above) not duplicate geometry I do them as multipolygons; and as such since there is usually one continuous stretch of grass along both sides of a highway it only makes conceptual sense to merge the multipolygons together into one for each section of the highway. 

For these reasons, and more, multipolygons are the best way to map land use and land cover in my opinion, and thus we should have /more/ of them, they should be the default in my opinions with the obvious exception of a single land use area not attached to anything else. 

Happy mapping,
Udar