I spend as much time trying to maintain the usefulness of SO as I do answering questions. To me, the best tool for the job is the mighty close button. By limiting me to 12 closure votes per day, you're allowing SO to get more polluted with superuser/serverfault/doctype/nonsense questions. What's the rationale?
Everything is rate limited.
The rationale is to keep you from spending too much time on Stack Overflow by limiting your daily actions.
If you spend too much time on Stack Overflow, you turn into a sad approximation to a man with over 8000 meta reputation points. Do you really want that to be you?
Other people have close votes and can pick up where you left off when you reach your limit.
This same reasoning is the basis for all of Stack Overflow's limits: voting caps, reputation caps, spam and offensive flag caps, deletion caps for 10k+ users, et cetera. There are enough users with the privileges to vote that the things that really need the votes get them in spite of rate limits.