Maybe for the judge, but not for people who don't like the theme, or for people who've already posted their entries. They're just sitting around bored.
I don't know any good reason to keep a contest going for longer than it generally takes people to think up a comic and write it. I've always said that quick turnover is important to Comic Contests.
On the one hand, there are your arguments, as well as the fact that leaving a contest open for too long makes people lose interest in the entries they did enter. When/if they win, they don't know for days because they figure they still have time.
On the other hand, some people don't visit this site every day, so having the contests open longer allows them to enter at least once.
Furthermore, what happened to the "good old days" when strippers used to try to top one another's entries? It's becoming less competitive with no one really reading anyone else's material. Maybe there's just too many people here now...
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Mediocrity at its most average.