In today's world of SEO being so competitive behind-the-scenes, bloggers have been coming up with better ways of getting exposure for their blogs through blog collaborations in order to get backlinks.
You don't want to overdo the promotion of your own site in an attempt to gain more exposure with their readers. The last thing you want is for them to be turned off by what they see when they get there.
If this is done in moderation then it's not really a problem. Just make sure that it's evident that even though they are on your site, there isn't any forced self-promotion involved in the way you wrote something up for them.
I've seen so many blogs have no problem spamming links all over the place without thinking about whether or not it's appropriate or relevant to the topic being discussed.