I agree with Ross that an awful lot of television shows -- especially the better ones -- would benefit from defined cutoff dates. To me, though, the issue isn't so much that some shows (The Sopranos, say) have overstayed their welcome by staying on the air (metaphorically, in this case) too long. Rather, the point is simply that good storytelling requires planning. If a writer knows he has 36 episodes to fill, he can write a good 36 episode storyline divided into three 12 episode arcs. But if you don't know how much material you're supposed to produce, or if you wind up doing an unanticipated season just for the money you maybe come up with something not so good or stretching your material too thin.