A disadvantage for the advertiser is that Google sees their ad as ineffective (no-one clicks it) and may discontinue it (at least at one point did they drop ineffective ads) — of course this ineffectiveness would apply equally to all ads (assuming everyone was using this plug-in).

Speaking in magic unicorn fantasy land terms:

If this were true and Microsoft made that a default feature on IE, what would happen? Assume that Google does not find a way around the issue (like I said, magic unicorn fantast land scenario).

umm.. yes. You are right. But this is not too different from adblock, which gets rid of the entire frame. I assume google is not too fond of that either

On the other hand, if X% of all surfers make such uncharged clicks, then advertisers will be willing to bid about X% more per click that is charged. So it may not make that big of a difference, in the end, to the monies flowing through Google.