Not only would it be impossible to see what your RP-Partner is saying in system if everyone was using it, but a lot of RP calls for some secrecy.
For example, when I'm on my Pirate Transport, pirating in say... New York. If I was to give my pirating messages in system, I'd have about 60 seconds from the time my first message hits the screen, till the indie capships start rolling in.
I tried system chat before on that character, it got a couple LABC's and a bunch of other indie LN's in GB's and fighters down on our party of my transport, and 2 GB's.
I actually make sure not to type anything in system on that character anymore.