Yeah I am relieved that it was brought up sparingly. It made the show even better. It was already great that in some of the episodes it wasn't a matter of zap and trap. Sometimes they had to get real creative in how they had to dispatch ghosts. One of my personal favorites is Bustman's Holiday. There were way too many ghosts to capture so they turned garbage trucks into Ghost Traps. How about the time they used a satalite to transfer the ghosts from the Eiffel Tower into their containment unit. Sorry I am geeking out hear back to the topic.
I believe that crossing the streams was brought up twice in The Real Ghostbusters which I think is pretty amazing with 140 episodes. They mention it in The Boogieman Cometh when they get trapped in that giant snake pit thing.
Peter- What happens when we blast this thing with our particle beams?
Ray- No telling. The laws of Physics are different here, it could be even worse than crossing the streams.
Then of course there was the Captain Steel episode.
Now I believe in the Real Ghostbusters Comic from NOW crossing the streams was brought up and even executed a few times during it's run.