Hi Tina,
Generally it is not good to add everything to main Lilypond — it leads to cluttered interfaces and becomes hard to maintain.
Maybe a good question for determining whether something should go into main LilyPond would be: Is this addition something that improves a shortcoming in LilyPond? If that is the case, I tend to think it would be desirable to get it into LilyPond.
Of course your point about cluttered interfaces and maintainability is valid; but on the other hand, I always disliked the situation in certain commercial software products where you needed plugins in order to accomplish even routine tasks or to get decent layout for (say) slurs and the like.
With Lyrics, my gut feeling would be that there's room for improvement in LilyPond's default handling; both Jean's code for spacing the notes without regard for the lyrics and the currently discussed snippet for eliminating spaces would be things that I would not too quickly rule out could be desirable as regular LilyPond features.
Lukas
