Unfortunately today I have discovered I do not enjoy vocaloids.
Maybe I need more immersion.
I would jump to the defense of vocaloids, but I frequently ask myself why I like them so much. Sometimes I'll go into my vast personal collection I have downloaded, and I'll start one up, and I'll ask myself "Why do I like having a modified Microsoft Sam sing me chiptunes in a language I can't understand?". Please allow me to use your comment to soapbox about my pet theories.
Reelya postulated that it depends on the composer, and gave some examples, though one of those examples was Ievan Polkka which is a stupid meme song, but his point stands. I have a feeling though that your post was in direct response to the link I had posted on the previous page, which was composed by PinocchioP, and this is purely my subjective opinion, but he's one of the best Vocaloid composers in existence, and if he can't win you over, I doubt anyone can.
So I've spent some time today thinking about it, and I suppose I can only speak for myself personally, but I think the strength of Vocaloid lies in its narrative possibilities. The music varies wildly in quality, and honestly if you're listening to the music raw with no contextual backing, it is easily the weakest part of the Vocaloid experience, even for the very good songs. I think the real strength lies in it's ability to be used as the backdrop for a wide range of stories and ideas. You see, the Vocaloids are just computer voice programs, but they have fictional characters they're attached to. They're canonically "Robotic Idols" or somesuch, but that's irrelevant, what IS relevant is that those baseline characters can be used for whatever the creators' songs need them to be. There are songs about them as ghosts, as murderers, as crazy stalkers, as friendly kami, animal hotel owners, as lovers singing a duet with eachother, as frenemies, as King and Queens, or servants and maids, or as personifications of the seven deadly sins, just a huge wide variety of creative roles that they've placed these characters into. I'd say this is where the largest strength of Vocaloid lies. The comparison to guitars was apt, but not simply in the musical sense of infinite possibility, but also in the storytelling perspective.
The Vocaloids have been described as "Immortal Teen Idols", though in the original context that was because they never grow old like actual teen idols, but the extra dimension to that statement is that they're immortal because they're simple templates for people's imaginations, which will always be churning out new and interesting things, and can use the Vocaloids as the instruments to do so with.
Please don't think I'm trying to change your mind, I think that'd be disrespectful of me, as I believe there are simply visceral elements to some things that are simply unpalatable to some people, and I don't think for a second that I can convince someone to overcome their own gut instincts on a matter. This is just what I've been stewing on today.