Banks have a rule of thumb that says 3-4 products and you'll be too commited to them to move elsewhere. A home loan, credit card, offset account/transaction account... Asside from the extra income, why do you think they like to flog insurance?
For the most part, offset accounts don't cost the banks that much anyway. Most people tend to spend the funds they have in them the same way the do if they live out of a line of credit account.
If people see they've got $2,000 in their offset account they tend to spend. People do this with their savings accounts as well. Most people save money better if they put some in their savings account and some straight onto their separate home loan. Out of site is out of mind, the banks know this and would prefer that your money is visible where it can burn a hole in your pocket, so they can charge you more interest.
Despite this offset accounts remain an extreamly useful tool for investors. You've just got to have a savings dicipline.