My main monoculture concern is simply the security one - that a sufficiently bad WebKit flaw could expose a huge number of users.
Otherwise, I don't disagree with you - the monoculture of an open source project is far preferable to a closed source one.
I also think that such monocultures - like the Linux kernel itself, of which there is only one but which is used by many different distributions - has shown that it can be reactive enough and diverse enough that the security concerns aren't much more or less worse than any other monoculture.
Which brings us back to the fact that Opera's action is effetively trading off the strengths of diversity for a hopeful strengthening of a monoculture. It's kind of easy to see why some people are a little uneasy about it.
(But as I've said, I'm for it - providing I lose no features in Opera!)