On the advertisers side your almost always going to want to get another ad impression counted rather than have the same ad displayed from user’s cache. Typical stats suggest something like 20% of page impressions are from people using the back button. That’s a lot of potential impressions to lose, compared to the very rare occasion that a user actually wants to look at the exact same ad they saw before.

Of course if your not rotating ads on the page in question, they’ll see the same ad creative, the cache busting will just ensures the publisher gets another ad impression counted.

I suppose the user’s browser could be configured to use only cached content when using the back button, if that’s the behaviour wanted. Though in a lot of cases they’ll want/expect page content to be updated, particularly on more dynamic pages such as News sites.