Have you ever had to create a conditional input field? For example, one field is a checkbox that reads "Do you have children?" and if the user selects "Yes", we want to show an additional input field that asks "How many?" (and if the user selects "No" the field disappears).

I ran into this in a recent project and, as usual, added some custom javascript using #after_build that did exactly that. I'm pretty sure this is a widespread practice.