In addition, its not really the indexing thats the problem, its being indexed and cached.
Where Google is concerned yes, but other wise no.

I can come to your site and request robots.txt. If I get 404ed then you "have nothing to hide" if I get the file then I can see what you are trying to hide so I will go and look at it. It is publicly available so you have it there for someone to see - just not the whole world.

It's security by obscurity in it's worst form since you actually tell anyone what it is you don't want them to see knowing full well that once they know about it there are no further access restrictions on it......