Messaging integrating with other apps?

Hi all,

Just a quick question: If you go into the messaging app and click the "+" to start a new conversation, you'll see a drop-down next to the word "People" at the top. My question is: why?

Could another messaging service (Skype or Gchat, perhaps?) let their app talk to messaging app and you could select a contact from that group? Was the messaging app designed to me more of an app to aggregate different chat clients?