There are workarounds, such as privoxy. Still it's a matter of priority, if you consider usage of plugins more important than 100/100 ACID3 score then you should pick such a browser, if you don't then go ahead and use Opera. More often I value the importance of licenses much higher than functions, except some specific cases. To me virtually nothing is more important than to have the entire product, including blueprints (source). There's nothing wrong or right about that, it's just the way I value software and it will benefit me in some areas and cripple me in others, which is why I try to be somewhat flexible. Being somewhat of a perfectionist I hate to make crappy adhoc fixes, which I would consider privoxy to be. There's nothing about privoxy that the browser itself couldn't do, so why not include that in the browser and make the browser better? And if somebody doesn't want to I either do it myself or , if I don't know how, ask for someone else to do it for free or for money, much like anything in life.
Basically I'm one of those whom always prefers open source for these very simple reasons. We who do don't do it "just because", and those who cannot understand why we do usually have very little coding knowledge, hence why they cannot relate to the benefits, which is fine if you ask me. We all make choices based on our own experiences, and if you've never benefited from a source code, or know that you have, it really does make sense.