The whole "keep it in private" thing is just frankly not that possible sometimes... If you say keep it in party chat: you have to keep inviting / leaving parties to join in to some people meeting up at the bismark. To use a linkshell means anyone who is in that linkshell but not there still gets the messages,and you'd have to leave the whole linkshell instead of walking away a little, and don't even start having two groups rp at different places because that would be just too confusing. In fc chat you would very likely be disturbing someone much more easily than in /say as not the whole fc might rp at that time and someone doing a dungeon foe example still gets all the messages, as fc chat is used for other things than rp even in fcs that soley rp.
Now with /say, you can just walk away or ignore it. I honestly don't get why you even let /say show in your chatlog if you don't want to hear things other people talk about. It's NEVER important things. Like shout chat in limsa. And if you say you want to be able to communicate with multiple people without forming a party or linkshell, while all being in the same area, congratulations, you got it!
Then there also is one more thing about the visibility of /say. We, as RPers, WANT to be seen, and for others to know we are here, especially on non-rp servers. We want to find other people enjoying the same things as us. You can tell us to look for people having te rp status up but most people who rp just use that while actually doing rp.

Now, one more thing about erp. I don't care if people do it in the game in general, but it should really be somewhere where nobody can notice. If you do it in your own house (squints at old roomie who erp'd in my house in /say) go ahead! Using /party chat? No prob x'bob tia! Standing in the middle of the quicksand fully clothed sending each other /tells? I couldn't care less, as long as i can't know what is going on between two consenting adults! Hell, even if the only ting you do in this game is erp with catgirls, as long as you're not rude about it, just do whatever you want!