The first part of your post, I agree with Hooty. But isn't the last paragraph a bit harsh?
At least they were willing to discuss the matter on here, rather than just go ahead and quit. Although, whether the original post was intended to "put the cat among the Pigeons" or not, I don't know for certain. Although I suspect it was (only 1 post, then nothing?

).
We've had some individuals (but not many), who've left the CWU, because they were so disilusioned. But they still haven't crossed the lines. They just didn't want to waste (their opinion, not mine!) their money on subs, when the CWU is so out of touch. But still stand by their collegues, metaphorically speaking anyway (they never turn up for the Picket Lines). Whether any of them have subsequently come back again, I don't know. But they certainly made a big fuss about quitting the CWU, at the time!
Now I don't know whether an entire office would behave in the same way, during a National Strike (if we get one). But telling them that they'll go to Hell for leaving the Union, is more likley to make them want to cause us trouble. Rather than change their minds, isn't it?
Why do you assume that they will be crossing Picket Lines, if they leave the CWU?
How do you know that they wouldn't try joining a different Union. There are alternatives to the CWU. They're not recognised by RM. But let's face it: it probably won't be long, before RM try to derecognise us anyway. They've been trying to ignore us for years, already.