So far, I believe I've only ever used it when talking about refugees, in the sense that I want all who are currently fleeing to have-successfully-fled. I quite like the contrast between -taH and -ta' and find it quite effective. Other times I've used something like {Haw'ta'wI'pu' mojjaj Hoch Haw'taHwI'pu'.}, but that's less punchy to my mind.

So, I suppose I'd use it when I think the desired effect is worth bending the rules, and otherwise not.

But only in an informal context, right? You wouldn't, for instance, send a report to your superior with your mu'mey ru' in it, right?

You wouldn't, for instance, submit a report that said Earnings were down this quarter because reasons. That's an example of a mu' ru'; that's similar to the effect of using an aspect suffix and -jaj together.

-- 
SuStel
http://trimboli.name