[fragmede]

It's because you don't want to leave a pile of shit for the next person to clean up. If you're maintaining code, and a function seems to be returning the wrong value for the ticket/bug you're investigating, do you comment out the whole function and just return $right_answer, or do you investigate what's going on so that the function is returning the wrong answer for a given set of args.

By that same extension, a custom hacked up theme is the "return 4; /*" of site maintenance.

By using a plugin, the person that follows you doesn't have to be a webdev who could float a css div blindfolded.