You might find yourself needing to apply to schools after first semester. A sudden GPA decrease after 3 years of good grades just makes you look lazy, unmotivated, and kind of immature. You're basically telling them that you aren't that serious about getting into law school, and that you might not have the work ethic to do well there.
Also, schools that have not yet made a decision with an application will see your grades. If they are markedly lower than your previous grades, you're probably going lose out on some acceptances you would have had were you not lazy and apathetic.
If you really care about going to law school, then don't screw up your last semester at college. This just seems obvious to me.