In a good PoliSci paper, you need to think of counterarguments for anything that you say and rebut them. Also, because you learn so many different theories, you learn to accept the fact that there are 2 sides (at least) to every issue. You learn that there is never a black and white answer. You learn that you need to use reasoning to prove a point in a paper. You learn to see everything from so many different perspectives and ask the very questions that come in handy in law school. Also, I think that cranking out papers all of the time for Political Science prepared me for law school. I have scored above average on every legal writing assignment this year.
? Am I the only poly/sci major here who didn't even begin his/her undergrad "poly/sci papers" until I ran out of beer the night before it was due?? I can honestly say, none of what this person said ever even crossed my mind while writing these things. I highly doubt any undergrad, at least the ones I know, approached their assignments in any way that would prepare them for law school (However, the copious amounts of alcohol consumed did prepare us for the second year of law school).
There were many times that I procrastinated a paper until the night before it was due (or even the morning of). The same has been true with my legal writing assignments, though Many of the undergrad PoliSci students I knew were quite brilliant (myself excluded from this). So the undergrad students that I knew actually did approach their assignments in a way that sort of prepared them for law school or graduate work.