I use the answer 10+15 phrase - isnt this good enough?
No, QA's have been solved
Bot users (mostly XRumer users) go back and look at where they have failed, they then answer your questions... these questions are then shared with thousands of others
Then all bot users can automate registration on your site
With QA, it is only a matter of time until a bot user decides to answer your questions.. when this happens, you get a flood of bots
You can change your QA questions regularly, but you will always be playing a game or Russian Roulette when you use QA alone, you never know when the bot users are about to manually solve you questions and have them shared