The ad network is a vector for malicious code transmission. To that end, an ad-blocker is exactly what you need. Your browser may be exploitable, but I wouldn't doubt that you can siphon up some very valuable personal information using the security model and API your browser implemented.

Just like UNIX-style permissions might, in theory, prevent an unprivileged user from completely hijacking the core system, the model says nothing about user-level applications manipulating user-owned data. I'd argue my data is much more important and valuable than whether or not I need to wipe and reinstall my OS.