From a MonoDevelop blog post:
There were several long time pending
bug reports, and I also wanted to
improve a bit the performance and
memory use. MonoDevelop creates a
Parser Information Database (pidb)
file for each assembly or project.
This file contains all the information
about classes implemented in an
assembly, together with documentation
pulled from Monodoc. A pidb file has
trhee sections: the first one is a
header which contains among other
things the version of the file format
(that version is checked when loading
the pidb, and the file will be
regenerated if it doesn’t match the
current implementation version). The
second section is the index of the
pidb file. It contains an index of all
classes in the database. The index is
always fully loaded in memory to be
able to quickly locate classes. The
third section of the file contains all
the class information: list of
methods, fields, properties,
documentation for each of those, and
so on. Each entry in the index has a
file offset field, which can be used
to completely load all the information
of a class (the index only contains
the name).
So it sounds like it’s really just an optimization. I would personally not include it in source control unless you find it makes a big difference to performance: my guess is it will only really stay valid if only one person is working on the project at a time. (If it’s big and changes regularly, you could find it adds significant overhead to the repository too. I haven’t checked to see what the size is actually like, but it’s worth checking.)