Well I admit, I am probably a bit biased by the fact that I was like 12 when it came out and was of course instantly and deeply in love with it back in 1993 lol. Having said that, I don't think the songs would have been good without Mariah vocals, as the songwriting is not super strong. However, I don't think Celine could have recorded it and made it as gratifying to me, there is just something so appealing about her vocal technique around this time.
I'll have to agree to disagree with you about Never Forget You though, I still love that song, but I am a sucker for Babyface too.