The rule was implemented because lyrics were often submitted in a very raw format. Very poetic, but not fitting in the song. So then Mike and/or JP had to rework the lyrics and add the melodies to them. So that's just a lot of work for them to do. I guess they just were tired of doing this work and then see the work credited to the original raw lyrics author.
That's still a lot less work than writing the lyrics and vocal melodies from scratch - even without melodies, JM writing the lyrics gives them another song that they don't have to write all on their own. So it's a timesaver either way.
My guess is that they didn't feel particularly comfortable editing JM's lyrics. To take all this wonderful imagery and stomp down on it until it fits in the box. Like if your friend had done a fantastic painting and it was your job to rip all the edges off. I figure it's more a creative and personal choice than a time-saving choice.
Keep in mind also that, by not writing melodies, Myung was being quite lazy in comparison to the others. Everybody else manages it. Myung's always portrayed as the victim in this scenario - because everyone likes his lyrics and it precludes us from getting them - but in fairness, the rule does stem from Myung doing less work than everyone else.
This doesn't have much to do with the new drummer, though.
I can understand the reason for the rule; poetry and lyrics are two very different things.
But ages ago, way before MP said anything about leaving, JM expressed an interest in contributing again, so perhaps he is willing to put the effort in and create lyrics from his poetry, rather than getting others to do it. I hope so anyway, he comes up with some great stuff!
This, too. Word for word.