Well it pretty much comes down to if you want your images to show up in Google image search or not.  Benifit would be that it might bring in some new people to your web site - but I could see the cons too.  To turn away just the Google image bot, you would need to create or modify your robots.txt file to include this:


Hope that helps!  If your looking for a good Robots.txt generator, I would suggest this one here:

http://www.mcanerin.com/EN/search-engine/robots-txt.asp