Muddiest Point 11/22/10

In class we discussed the z39.50 search.  I also remember seeing this in our Koha project assignment.  In the slides it stated that in general these types of searches are not used due to the fact that they are difficult to implement, can have semantic problems, and network server problems.  So then who really uses it?  And why would they use it over another type of search, especially since it does not retrieve full documents or other objects?