The only time you wouldn't want a Recommends: installed is if you know that it wouldn't be useful.
Not really.Policy is vague and only specifies that Recommends means that more than a proportion between 1/2 and 1 of systems consider that the recommended package's contribution to the recommending package makes the recommended package worth installing if it isn't already installed. Suppose that proportion would be 1/2. 1/3 of systems initially without package A install it when installing package B. Package A should therefore be suggested by B. 2/3 of systems initially without package C install it when installing package D. Package C should therefore be recommended by D.
Both systems that have B but not A and those that have B and A have their reasons. You'll agree that system administrators installing B that do not know why B suggests A can still want A's contribution to B. In the same way, both systems that have D but not C and those that have D and C have their reasons. Administrators installing D that do not know why D suggests C can still want C's contribution to D.
In practice, the proportion for Recommends is higher than 1/2, but the same reasoning applies.