I haven't written a plugin, but I don't find it hard to come up with reasons to write plugins:

• You want a different syntax (perhaps to have identical syntax as language X).
• You want different rules (for instance, POSIX's "longest match" preference over Perl's "first match")
• You want an engine that's optimized for certain cases. Perhaps you want a pure DFA - sacrifizing functionality for speed.
• You may want to do matching in a particular encoding.

I've no idea whether any of the plugins have been written with those reasons in mind, but I wouldn't be surprised if someone at sometime does.