It would be nice if it was that simple, but if you accidentally word something the wrong way you can open yourself up to a mess of problems. Employment lawsuits and accusations of discrimination can be very nasty and costly.

If this wasn't a concern, I would gladly take the time out of my day to write a nice note like this. Even if they're not hired, many technical people will tell their friends about their good experience with you and help improve your reputation in the tech community.