object placement. [...] avoid placing objects on top
of each other.
Exactly that can (of course not for all purposes) be an advantage instead of
a disadvantage. In mapnik, often information is simply omitted due to overlap
avoidance. This is bad if I am interested in a special topic:
If I am interested in parking, I don't want that a parking garage is left out
because there's a nail polish shop in the first floor of the same building.
Of course it would be good if the user could sort and stack the layers according
to her priorities.