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Eh... saying how it's better...

- Not everyone who makes music can sing, but the Vocaloids can [also very familiar to the composer if they're used to synth-type music]
- Also, you can compose the vocals exactly how you want them to sound, I think sometimes it's harder to get a real vocalist to do that
- It's a niche culture, so even if you're just starting out, your music has a chance to get noticed by those who already love the software
- Even if you do have a singer for your song, they make pretty good backup vocals [LOLA is in the original Byakkoya no Musume, for example]
- You can do things easily that would normally take a lot of voice-editing, like the blatantly-robotic voice in kz's remix of Love is War
- And the obvious; they can hold insanely long notes, and can go as high or as low as the composer wants [and don't necessarily sound terrible]

For realisticness... definitely Meiko's or Miku's version of Dearest, Kaito's Byakkoya no Musume. Uh... I know Yuyoyuppe has made Vocaloid screamo, using some separate voice-editing... like in For A Dead Girl+ or ALONE. That's a good con point for Vocaloid, though, you can't get screamo very easily, but it can be done.