I don't know, maybe its better to leave as a plugin.
The advantage of being a plugin is that if I want to add another export format I'll don't have to deal with C::B main code. In fact, I'll only need the SDK for doing a change, not the entire C::B sourcecode.
I think it's easier to make a change (ie. patch) for a plugin.
Keeping almost all as a plugin is very welcomed these days in the IDEs.
One of the major claims in advantage of the Eclipse guys is that the IDE is very extensive through plugins, every language supported is a different plugin.
There are advantages in merging a plugin in the main code, but for the type of plugins that tries to "fix" something from the C::B main code, like the FileTabMenu plugin.
EDIT: I've obviously misread the post
Regarding including it in the built-in plugins, I'm not sure, but whatever plugin the core-devs aren't going to mantain it, it's better to have in the contribs then.
The disvantage of the contrib plugins is that they maybe will not be compiled and included by some of the linux package mantainers in their C::B packages.