There are two reasons for that:
1) I just don't feel ready to code directly into VLC. Or at least I don't know the formalities of contributing to VLC. So I think, a plugin would be an easier way for a proof-of-concept.
2) I want to display lyrics (among others) in the "Media Information" panel. My current approach is to put it into the "Extra Metadata" tab, but that is ugly. I am currently working on a demux plugin for the MPEG-21 File Format, or, to be more precise, a hybrid version of MPEG-21 FF and MPEG-4 FF. And these audio files contain lyrics. I only want to display the lyrics for this file format, so a full implementation into VLC is out of scope for now. I found that also other people are asking how to display lyrics (
Topic: playlist feature requests), but there seems to be no solution yet.
[EDIT: So, the plugin I am proposing would show the "Extra Metadata" tab with a tree structure and would add a "Lyrics" tab to the panel.]
[EDIT: On the length limitation of the mentioned items: Could that be a bug or limitation in QT?]