I have Bastian and Mirri as companions to my main character, fully leveled up and with purple gear, but it wasn't easy to get their gear to that state. I was recently considering getting 2 of the other companions, 1 each for 2 of my alt characters, but was feeling discouraged from doing so, and the main reason is: companion gear cannot be upgraded. It's a major grind to collect companion gear in reasonable quality, so the companions can face a reasonably tough enemy without being killed quickly, and buying companion gear is very, very expensive (not surprising: the gear is a grind to collect, and this pushes up its selling price).
Being able to upgrade companion gear would make adding new companions a much more attractive prospect.
PC(Steam) / EU / play from Melbourne, Australia / avg ping 390