Some sites that do this are looking long term... But I won't get too much into that.

We recently converted one site to heavily promote other sites and almost immediately increased our revenue many times over without loosing any of its direct or affiliate sales. It's been so successful that we are now starteing to do the same thing on the straight side.

I think when done properly it can definitely be a huge boost.

On one of our other sites we promote other related sites not so heavily but enough so that it generates a decent amount of revenue. Plus the sales we've been seeing off our exit traffic to related sites is a huge boost.

Competition is great especially when its mutually beneficial.