Sorry, I don't agree: if you're dealing with user input in a situation where they could (intentionally or unintentionally*) damage the system and the language offers you a helping hand, why not use it?

Yes, but that's a far cry from "having it on all the time". Most users cannot (on a proper system) damage the system anyway. It doesn't make sense to have taint checking on oh, say "ls", or "vi" (unless you were to make a restricted shell inside "vi").

And it's not that the language offers you a helping hand free of costs. It's like equiping electronic locks on all the doors in your house - including the doors to any closets, and the lid on your toilet. Locks that can only be unlocked (for one time usage, closing the door auto-locks) by typing in 16 character hex code. Sure, it helps reduce theft from your home. But it isn't very convenient to go from one room to another anymore.