It's not the developers' jobs to make their sites work with Opera (especially when it works in IE, Chrome, Firefox, and Safari). It is Opera's job to make sure they render sites comparably to other leading browsers.
If that means they rip out their rendering engine and replace it with Webkit, so be it.
Go through them and let me know if you still believe that Opera doesn't try hard enough. The problem is that being standards complaint is harder than it soudns. A lot of the nitty gritties aren't explained in standards and diff browsers implement them in diff ways. While developers explicitely implement patches for Fx and IE, they generally ignore Opera. And that's why so many websites don't work in Opera.
Coding wrong and passing the problem to the browser is just mean
BTW, different rendering engines, assure no monopoly, innovation and competence, which is always good.