Wow! Interesting thread. Well are they? As should be obvious by now, that depends on the intent of the recording artist(s) and the recording engineer(s). I think with some artists, the intent is precisely to convey meaning through lyrics. This can be done effectively with some awful singing (Bob Dylan has been cited above), or with really good singing (Freddy Mercury has also been referenced). Sometimes the vocal really is just an instrument. The logical extension of this is Ella Fitzgerald's scat singing.
Sometimes artists change paradigm and intent over time, resulting in the difference between "I Wanna Hold Your Hand" and "Back in the USSR". I think the real problem arises when the mixing and mastering engineers don't get it, and either place meaningless vocals up front and center, or they mix the lead vocal in a ballad down to the point of unintelligibility. That was my complaint with the first Nickel Creek CD. They mixed Sarah Watkins' delicate voice as if it was an instrument. It sat perfectly in the mix, and the story was lost completely. If you lose the story, you just lost David Wilcox and Maddy Pryor.
Often the balance between the story and the mix has to be a compromise, as with something like The Who, The Beatles, or The Eagles. In most of Country music, the story is a priority. In most of Disco, although it was vocal based music, it wasn't particularly important. Me, I'm a storyteller. It's what I do. It's all that I do. If a mixing engineer mixes my songs, and the lyrics are lost, I'll get a new mixing engineer. What does this mean to me? KCearl probably isn't going to buy my album. I'll sit up nights worrying about it. But it also means that when I mix something with vocals in it, I ask myself- does the story matter, really? If it does, there's going to be a little midrange boost applied, and the level of the lead vocal is going to come up until you can understand every word. The worst case scenario, of course, is when the diction of that lead singer sucks. Then you really are mixing Bob Dylan. When it gets to that point, you hope the story really is worth it. In the case of Bobby Dylan, it was.-Richie