Honestly, and there is no shade intended, any of Ne-Yo's songs could be re-recorded by another artist and likely go #1. He is more of a songwriter than a singer now. He's always written amazing songs, but I'm noticing alot of people aren't into his music when he's singing it. If he had kept Irreplaceable for himself, the track would have gone to waste as casual listeners would not want to hear a man telling his boyfriend to go "to the left, to the left".
He definitely did the right thing by giving it to Beyoncé. I don't think anyone else, not even Rihanna, could have done that song justice.