the reason so many melodies in R&B are so weak today is because they are written from the lyrics. that's why you get this very popular 2- or 3-note stutter-step "melody" (think Destiny Child's "say my name") that all these young artists employ

i think when you conceive the melody independently, you have a better chance to come up with something catchy and fresh and (most importantly) musical than trying to fit notes onto words you've already written.