You are absolutely right to be selective about how you approach material concerning the Rain Robinson character. If you never do include her more, well ... at least you have and it makes perfect sense for her to be in The Eugenics War novel. She was around during the decade when Khan was, apparently, up to his mischief. So, there is a connection there that's in no way "tacked on." It's cool to see someone who's much closer to the franchise than I'll ever be who also thinks Rain would make a great subject for a book ... as much as I. So, absolutely, get the word out ... Rain is still in the TREK universe, having adventures! Don't we all wish we were, though?