Interesting thread. Of course a good melody is better than a poor one, but sometimes I find myself paying more attention to the tune than the lyrics, so maybe there's an argument that if you have lyrics that need to be heard, you should keep the tune simple, so your audience will pay full attention to the words. I don't know whether any songwriters do this consciously though, or whether it's just that they don't know how to write good tunes.

I agree there are too many songwriters that write 'nothing' melodies, or very derivative ones that leave you yawning inwardly. (But I'm not much of a songwriter myself, so maybe I shouldn't be so critical of others).