I think there's a difference with alot of the names mentioned:
Kelis - has always been eclectic!
Rihanna - has always been eclectic. Her albums have always been very fused, even as "her sound" has matured, she remains fused. She's a pop star (in the general sense) but her "music" is what it is from song to song.
Usher - used to dominate with his more R&B music ONLY to both pop and urban audiences. As music, fans, industry and radio became more polarized he had no choice but to cater to his urban fanbases and also to his pop fanbases.
Ne-Yo/Chris Brown - trends when it comes to this!
Beyonce - has always had fused albums and, like Usher, used to smash with singles that were very urban, yet crossover. Now, she can't pay a person to buy her urban music right now, but if she releases a IIWAB, Halo or SD, they'd be running!
That's about it. Everyone else is primarily singing straight pop (Gaga, Perry) or purer R&B (Mary J., Monica).
Now, the rappers (particularly the Cash Money crew) don't necessarily live by the formula. The label has ALWAYS been album-driven, and not single driven. So, they release buzz singles, jump on collabs, etc., THEN pick ONE urban song that is supposed to be "their smash" and promote that while they release ONE crossover (Wayne - How To Love, Drake - Take Care) that pulls in the other casual (pop) buyers. They ultimately sell lots of albums in the US.
I think it's necessary or we'll have no Beyonce's and Usher's seeing success, and everyone will be the next "LMFAO, Derulo, Flo Rida, etc."
"Take A Bow"... Rihanna's ONLY ballad to go #1 on the Hot 100 (also topped the R&B Chart)... Great deed for her most epic ballad!!!