It has actually backfired on me in the past: because I didn't define and explain the entire space of the matter at hand, I was overcome by other arguments or ideas that I'd already considered and dismissed. That led to followup discussions that didn't need to occur.
It seems that context matters, sometimes long-form is better.
The problem seems to be that it's a judgement call - and I'm not exactly sure of all factors/variables that are involved in that judgement call. The subject at hand, the audience, the depth of the e-mail thread to which you're replying, time-sensitivity of the subject, etc. - they all play a role. But my default position tends to be that I'll save time explaining in future e-mails by providing all relevant details at hand now.