First of all - it's illegal! It's stealing! And not like stealing from Walmart, who make enough profit to not really care about a theft here and then. It's like stealing from your local private-owned shop - taking the money away from somebody who has to feed his kids from the stuff he's selling!
Second thing is: you are running the risk of it having a virus. Plus, you don't receive updates unless it gets uploaded, which means you are at risk of being exposed to a security exploit in an app for much longer.
And last but not least: the Nexus 7 is so cheap cause Google sells it just above manufacturing cost, with the aim that they get the money back from App sales! So you would practically make Google rethink that decision and never ever release another budget device! And others could follow.