Why are dirt and bugs on an aircraft so bad?
For (modern) gliders with highly optimized laminar flow airfoils, bugs (or dirt) are a non-negligible factor, especially in competition gliding. There are some older laminar airfoils that are notoriously allergic to bugs (and rain), but all are to a certain degree. This is also the reason why any glider pilot worth his or her salt will painstakingly clean their ship each afternoon and before each competition flight.

Sufficient bugs will degrade your performance by a noticeable amount - most gliding computers even have a special "bug" factor to include that into glide path calculation (even though the measuring and correllation of that is quite subjective). I´ve had a few flights in somewhat biblically bug-infested conditions (summer, hot, low thermals) that more or less painted the leading edge black (like in this picture from an article on the DG Flugzeugbau website).



Bug wipers are quite common on high-end and competition gliders and can be found e.g. at http://www.storka.at/. Or try to google for "Mückenputzer". Of course bug wipers are not without drawbacks (concerning cost, installation, ease of use, drag etc.). A friend of mine once stuck a banana peel on his wing and tried to remove that with the wiper, resulting in banana and wiper being stuck in the middle of the wing. It took some creative maneuvers to clear that mess, by which time probably a lifetime aerodynamic benefit of cleaning had been spent.

As for the comparison with cars, their aerodynamics differ in a lot of ways. In comparison to a wing there is no laminar flow to talk of at higher speeds (most of it is turbulent), which drastically reduces the influence of the surface smoothness on the drag produced.