If you make sure that login/registration pages have no ads, that's not enough to be secure. One example: you've probably clicked the "login" (or register) link

If you make sure that login/registration pages have no ads, that's not enough to be secure. One example: you've probably clicked the "login" (or register) link from a page that does have ads, and a malicious script could've hijacked that click and presented you with a perfect replica of a login (or register) page, and then captured your input. And I'm sure there are many other such tricks.