If you get an incorrect address, it's because the user made a mistake. Chances are, most of those mistakes will still be RFC-compliant addresses (wrong or missing letters, for example). I find it hard to believe that people inputting addresses without @s or with unicode characters would become a real problem.
Frankly, your lack of belief is likely due to a lack of experience with user submitted forms with email addresses. It's VERY common for users to simply type the wrong data into a particular textarea. If you do no validation you will get things like the person's name, street address, or other confused mixups.
Anyone who deals with forms of this nature will have seen this firsthand, and with enough frequency to cause trouble with mail relay as the person above has described. It's a real problem.
