We do direct postal mail advertising and use special URLs on most of them. (www.example.com/word) These urls redirect to the product(s) being advertised, and it usually works well. We don't use them for tracking, since we have separate codes for that, it's really just so that customers find what they want quickly.
Of course, there are a few users who type the URL into a search engine instead. They get more misses than hits, mostly "did not match any documents". But on a couple of them recently, Google's results lead to an "adult" site - nothing to do with what we offer. We didn't realize that until we got calls about it.
I hate to drop the custom urls, because I think they are helpful for the majority of our visitors. Is there a better solution?
If you got those urls onto the domain they point to somewhere you would be bound to outrank some random adult site for them.
Put html copies of your marketing letters online somewhere, or add the alternative urls to your site map - anything as long as the url is displayed in text somewhere spiders can find it (but link them to the standard url to avoid dupe content issues).