TBH I think that SE still handles it well compared to other MMOs I played. I can't imagine how to fight bots well thought. Also if they changed whisper function it would probably more affect normal players than bots/ gil sellers. They will always find way to overcome it and the only difference after 1 month after implementing it would be that people would be annoyed and would use /tells less.

I have seen mmos implementing even captcha ingame to fight bots and it turned out like a hell for normal players.