Using taint mode lets you to be future-proof. It's like using strict: you don't really need it (in the sense that you can perfectly do without it), but you understand that coding under strict mode lets you sleep quietly.

What if your program will evolve in time just to add some interactivity, or flexibility? You're already doing 99% of the work required to use taint mode: you're checking your input. Tainting it will help you not to forget anything, and will watch for dark corners you could overlook.