Of course there is still a lot of old web pages written back before the more advanced ways of interacting with the page was introduced and it took some time for those sites to update their code to use the more advanced and less obtrusive ways of performing those interactions (many smaller sites still have not done so). Perhaps the most significant change in this regard was when Google finally updated their ad scripts to use a more modern approach.
Turning off document.write in your browser should therefore be possible without it having any noticeable effect on your browser use except for making it easier to identify some of the worst designed pages where the page is likely broken in other ways anyway. I think it is therefore reasonable to say that we could turn off this command in our browser (whether we should or would is a different matter).
This article written by Stephen Chapman, Felgall Pty Ltd.