The point of attestation is to verify the integrity of your execution environment. With a "compromised" execution environment, access to websites could be blocked. Presumably, the attestation process would send a fingerprint of your browser configuration to the attester, who would then be able to see whether you're using "compromised plugins," and deny you access by not attesting your browser.
There might be ways to filter away the ads after they've been served, such as memory manipulation, but the problem can't be solved with a plugin anymore, as browser attestion could let websites deny you access altogether if you use a plugin they don't like.