To be fair, the very obvious answer to your question was WAR, lol. If the guy's positive offense outweighs his negative defense, or his positive defense outweighs his negative offense, he'll have a positive WAR. It it doesn't, then it will be negative.
If a guy is a +4 hitter and a -2 defender, he's a +2 over all, and if he's a +2 hitter and a -3 defender, he's a -1.
Weeks, who's bandwagon my ass has been firmly planted upon for quite some time now, has more than enough offense to make up for his poor defense.
If you want defense, then play Goins. You're not going to get a great all-around 3-WAR second baseman now. Value is value, and Weeks will give you value.
At least, he should.