Or am I thinking the wrong kind of messages here?
Wrong kind. I'm on about in-mission dialogue. Every spoken line you'll hear comes from one of two sources. Either you have dialogue that the mission designer added themselves or you have built in messages which are defined in messages.tbl. Built-in messages are played whenever the code feels that they are needed. So if the code notices an enemy fighter on your tail it will send a message and one of your wingmen will shout something like "Check your six!" or "Behind you, pilot!"
Built ins are easy to recognise, any time you hear a voice in a mission that doesn't have voice acting, it will be one of FS2's built ins.
What I'm asking is which others might we want to add in the future. That's nice from the feature standpoint but it's also good for mods and TCs to have an idea what might be coming in the future so that they can record their voice acting now to support it. Adding a couple of extra lines which are currently unused isn't going to cause huge problems but they'll be glad they did it if the engine ever supports them.
An example of this is the Oops 1 dialogue.

recorded every single pilot shouting something in cases of friendly fire but eventually decided to not use them. When I was solving another issue with built-ins I noticed this and turned them back on. As a result you no longer only hear "Invalid target! Hold your fire!" but also a whole host of new messages which didn't play in retail.
Similarly if WCS want the SCP to add a message when a ship goes into autopilot or something (Not that I'm promising anything), now's a good time to ask.