Well, I also agree with all of what Bizarro said except for this bit:
Why doesn't it matter if you are singing by yourself? A song sounds different in different keys, the mood can change drastically."- When you sing acapella, you don't have to pull out this dorky whistle to hear the note first. "
So? If you're singing by yourself, it doesn't matter. If you're singing with others then you'll need a reference note for EVERYBODY to hear. It doesn't have to be a whistle...
You could be singing with others, but the song starts out acapella with the band joining in later in the song. Unless the singer uses a referece before starting out the song, they better have perfect pitch. Of course, its not really a problem using such a reference, so the use of perfect pitch there is debatable.
I don't think any skill should be undervalued, though, and I am sure perfect pitch has its great uses. I just can't think of very many right now. Oh, how about a piano tuner who lost his forks?
HOW TO ROLL A BLUNT