As with any unofficial root/affix, use with caution. I asked two native kids about "ri" and they did not understand it (they had never heard it). They feel it is a pronoun, but have no idea of what it means.
There is an Esperanto for reduced communities (for example interest groups, some families) where many experiments are made (ri, ŝli, na, iĉ ...). In a more global setting though, talking to (or writing to) other people you don't know, I'd highly recommend against "ri", "na", etc.
Why? It just hinders communication, and that kind of defeats the purpose of Esperanto. It's like willingly using your local slang with people that share your language.
Anyway, if you feel the urge to innovate, and you firmly believe in that new feature you (or somebody else) came up with, go ahead and use it, it might even be fun to try it (in our family we every now and then use "ŝli" in jest, for example). But be ready to fall back to the common "normal" international Esperanto if you want the communication to flow.
So the general answer to when you "should" use it: never. You "might" want to use it though in some situations, as explained above.