If you have a margin account, then I believe your shares are always being loaned to other members of the broker whether you like it or not and you get nothing for that. Under IB's stock yield enhancement program, your shares are loaned out to people outside if IB in exchange for some cash collateral and interest. From what I have heard and personal experience, it works out well except that in a taxable account, you might be paid payment in lieu instead of an actual dividend which has negative tax consequences for long-term holdings. Also, because I generally hold S&P500 stocks, the yield wasn't much. Probably a lot more if you hold more speculative assets that are hard to borrow.
Doesn't that mean that if you have a margin account, you should always enable share lending?