the reason your kpop fave may not get many lines in their group's songs, despite being a decent singer, could be that they're not korean but are singing in korean. yeah, non-korean idols may not want to sing much in a song because the pronunciation of certain words may be too tough for them - shuhua literally talked about this wrt her 'my bag' rap. and even if they don't have that much of a problem, and are willing to sing it, the producer may not want to give them the lines because they may have a thick accent. and while that's perfectly okay, it can take a listener out of the song - and pop producers want as many people as possible to listen to their songs, right?
just a thought.