Is smoking unfiltered cigarettes significantly worse for your health than smoking filtered cigarettes?
No. Filters don't really affect your health, except in possibly negative ways, which others have mentioned.
Filters were really invented to get ladies to smoke cigarettes. Ladies did not care for the fact that their fingers would become yellow from the smoke, nor did they enjoy getting tobacco in their mouth, which made it necessary to spit. Spitting is not very lady-like behavior!
The choice for ladies with a little class back then was, carrying around a cigarette holder, (you've seen them in old movies), or not smoking. Cigarette holders needed to be cleaned, and they stank up your purse. Filter tips eliminated this problem. They functioned as disposable cigarette holders. Who cares if you get lipstick on that?