If I were starting research into a field, or had to write a survey paper as a project, or recommend a breadth-oriented course of study to a student, I might do a search based on MSC codes. Usually my research interests are more focussed. Also, there are other ways of searching that would help the projects listed above.
One place where the codes might be useful is in generating glossaries by subject area. One could use a text processing system to form data points by code, number of occurrences, and so on, and use other means of preprocessing to come up with a reviewable list which could be hand annotated. I think a thesaurus for each major subject area would be quite useful, especially if it indicated historical usage.
Gerhard "Ask Me About System Design" Paseman, 2012.05.11