I've not found any valid reasons for linking to a competitor when it comes to building relevance.
I remember all those topics about providing links to your competitors. I stayed away from those because it was clearly an attempt at manipulating PageRank and relevance. I don't know where that suggestion started but I can only assume that it was started due to Google's ranking factors. I believe it was due to on-topic links that people thought it was a good idea to start linking out to their competitors. From a marketing standpoint, I personally don't find that to be a good idea.
lol, try doing that in a traditional marketing environment.
P.S. I always get a good laugh when I find an SEO site that has a page linking out to their competitors or those who have participated in your standard link exchange program. What a tacky move that is. :)