Many fans of opera agree. One thing that seems likely based on reading about opera of the past, and even some early recordings, is that singers used to have less vibrato, just like orchestras used to play with less vibrato. You shouldn't sing with no vibrato at all, but some of the best singers control their vibrato and don't make it too wide or noticeable.
I think it's typical not to use (much) vibrato when performing renaissance or medieval music, almost all of which is vocal. On the other hand, opera didn't really show up until the early baroque (Monteverdi et al.), so unless you can produce your own and instruct the singers accordingly, you may be out of luck there.
I have devised a most marvelous signature, which this signature line is too narrow to contain.I like vibrato. Even really strong vibrato. As long as it's being done right, otherwise it sounds like crap.
I am trained classically in singing. While I agree that vibrato is essential in opera, and that it varies in modulation and pitch, it shouldn't vary that much in both, or else it'd just sound absurd. Some opera singers do that, unfortunately, which (I'm guessing) caused a lot of people to have the impression that opera singers use too much vibrato. There's no such thing as too much vibrato anyway, there's only people who can't control them.
I don't much care. On the one hand, you can overuse it. But on the other, this is one of the ways for opera singers to show off their talent.
Except for 4/1/2011. That day lingers in my memory like...metaphor here...I should go.