Because sometimes you want to send people over to another site, but Google doesn't want you sending linkjuice. As per Matt Cutts, Google is probably going to take action against guest posting sometime in 2014. That doesn't mean it is a bad idea to guest post; it just means it is a bad idea of doing it to build links instead of introducing yourself to a new audience.
I just hate installing a plugin to do just one tiny thing. I have too many plugins slowing down my site as it is. The lower level something is implemented, the less impact it has on a site.
To cover your concern of confusing less technical users, possibly it could be labeled as "advanced" or with other disclaimer to only use it if you know what you are doing.