My thoughts on Vocaloid

Vocaloid is definitely an acquired taste, and I personally enjoy covers over the original songs. Regardless, they help in allowing new composers/producers to create full songs with vocals, even if they have no singing abilities themselves. From there, the songs can take on human vocals, with the many amateur singers there are on nicovideo. If you like a song but dislike the vocaloid singing it, there's almost always an abundance of covers to look at.

Heck, I know several people who are perfectly competent in singing, yet have composed several songs using Miku for the sound of her voice rather than theirs. Luschka is a perfect example. She's covered several of Treow's songs among others, she was the lead vocalist in his album, Reversus, and she's even released her own album, Birth and Foster.
http://www.nicovideo.jp/watch/sm11419786
http://www.nicovideo.jp/watch/sm11280964
Regardless, she has actually recently started composing songs using Miku rather than her own voice.
http://www.nicovideo.jp/watch/sm12623948
http://www.nicovideo.jp/watch/sm13125524

Though I can't think of a way to fully explain this myself, I believe it's simply because it's an enjoyable community to be in. Many of the big producers on nicovideo know each other very well, and the community as a whole is incredibly tight-knit. People can experiment with vocaloids, tuning them in different ways to see what sorts of sounds they can make, and they have the experience of watching as the vocaloids themselves begin to grow better and better, approaching the level that human vocals are at. But anyway, that's just my two cents.