Well I'm not sure that we are disagreeing. :-) My point was that the two tend to lean different ways, not that well designed software wouldn't use the most appropriate form of relationship regardless of its intent. Also I guess I was griping a bit, has-a relationships are IMO underused, and IME often more useful than is-a relationships.
Further to my original point, I think of lot of programmers, especially less experienced programmers (but not at all exclusively), tend to forget that OO can be used simply as a form of encapsulation that makes organizing their code easier. They get so distracted by all the subtleties of inheritance and overloading and polymorphism that they forget that OO can be used in simple ways to achieve complex goals in manageable ways. In fact one of the reasons I like perl and its anonymous subroutines/closures is related to this. Its a simple way to define what is more or less a single method class/object that encapsulates its data conveniently. The classic case is a counter object versus a counter closure.