As I've already mentioned (and I believe other people have as well), I don't like seeing those 404 errors because it makes me think there's a broken link somewhere, either on my site or someone else who is linking to me. Same goes for that stupid favicon.ico file that everyone requests - I uploaded a very boring icon just to stop hundreds of 404s clogging up my logs. Because it's quicker to type 'touch robots.txt'.
I stick at least a "allow everything" robots.txt file up to slim down on my 404 count in my stats. The annoying bots don't follow the rules, but I have a few uses for robots.txt - like stay out of my includes directory, leave my cgi dir. alone, etc.