I don't like customized environments. When I used Opera, I didn't have to configure anything, I liked the defaults and I liked that each instance of Opera, anywhere, was the same. Browser extensions make each browser instance unique. (For the same reason I don't like heavily customized vi/emacs/unix environments).
I also view browser extensions as something that ameliorates the perceived defect, rather than fixing the systemic cause of the problem. I think that browsers should come with ad blockers by default, and if I install one myself I'm always reminded that browsers are not like I feel they should be.
As a programmer, I like clean, small interfaces and good software architecture. My opinion is that putting ad blockers on top of the browser is bad design. I feel like filters like this should stay behind the browser. The fact that browser extensions are browser specific reminds me that extensions don't use a clean, composable, generic interface, but a browser-specific, ad-hoc one. I don't like that.
I install very little software, only what I really need. I chose my software carefully. The decision to install a browser extension is more costly to me than the decision to install a particular browser. I don't necessarily think the people writing the extension are as competent as the people who write the browser, and even if they are, they are more people for me to depend upon. I did try browser exitensions, once, and I learned not to trust their release cycle.
It's all extremely subjective.