the preprocessing hook makes sure that no infinite loops occur by restricting the depth of nested directives to 3.
this is insufficient in some situations in which sidebars are conditionally assembled from templates.
given there are no limits on the number of directives per page and the number of edits a user can do in a particular time frame, i assume that raising that limit slightly won't make the DoS attacks that can be done against ikiwiki too much worse.
i'd like to suggest 8 as a new value for recursion depth limit. most people can wrap their minds around a depth 3 nested directive setup, but when you reach a depth of 8, it's likely to be easier to write a dedicated plugin.
