Thats just it, if everyone went with your logic and no new products were developed because "Everyone just uses opera", then we wouldnt have some of the ideas we've had thus far in any product.
To me, opera is an incomplete and unusable product because I cannot make it "My own" by extending or adding on to it. In opera, I am limited solely to what the developers feel I should use. I have made a huge investment in some internet addons that, while opera may have a similar functionality, it doesnt compare to what I get out of these programs. Roboform, for one, puts the wand to shame. Again, if no one challenged what was already in existance, then we would be stuck with opera's limited implimentation and we wouldnt have the great program which is roboform.
Now, you claim more bugs, this isnt a real issue I think. All software has bugs, by saying we shouldnt have a CHOICE because of this is simply ludicrous in my opinion. Thats like saying Toyota shouldnt make more cars because there will be the possibility of more bugs.
More security holes. This is subjective. While more holes might be found, the number which actually affect users is another story. Out of the several hundred bugs I've seen on windows and with IE or Firefox, only one bug has hit me and that was blaster. So again, an exploited security hole is much different than one which is just "known".
From my understanding, no browser has yet to fully support CSS in the ways the W3C intends it to be, so this argument doesnt hold a candle, IN MY EYES, until one browser successfully does this and does so COMPLETELY without exception.
I dont know much about DOM, so I wont comment on it.
On your last argument, this goes back to my first point. Why should we limit debate just because something exists which is "Good enough"? Debate brings about change, and brings about new innovation and ideas. Opera leaves a lot to be desired, as do most browsers, so again, choice is good in that the users can pick what suit's their needs best.
It seems to me the major problem here, from what I see in your post, is that developers dont follow standards and nor do browsers (again, developers). I dont think we should stifle innovation or stifle giving the user more choices because we cant get standards agreed upon. Then again, I dont think web standards will ever be truely standardized simply because we are human and someone else somewhere will always think of a "better way" to do something.
So back to you point, I think I have shown why we cant all just "use opera". To me, opera limits my ability to work by limiting what I can do with MY BROWSER. Opera limits me by making me relearn certain basic concepts about a browser which are, for the most part, a standard amongst other browsers.
So again, more choice == better. Without it, we wouldnt have what we do now with many things in life.
Now, onto safari. Here are a few annoyances I've found.
No CTRL+TAB/CTRL+SHIFT+TAB to switch between windows. No ability to minimize the application by clicking the taskbar icon. No extensibility (A must in my eyes) so I cant use my programs which I've invested in with both time and money, inside of safari.
It seems like a good browser, and if they impliment a good extensions model or API, then it will be great and a real option for most users.
