Spambots can easily get around that. Now what's become popular is a simple question. You can even use both (of course, having real people answer the question.).
You right it's not completly fail safe but... what if we had a random question asked everytime you sign up at KIC, asking you some sort of park or site related question (some fact easily found of course), got the question wrong and you couldn't sign up. You can also take it another step and take pictures of rides at KI and have people signing up for KIC answer which ride it is. Since there are so many various angles a ride's photo can be taken in it would be very hard for the spam bot to guess the one, and then again got the picture wrong and you can't sign up