Getting your point across quickly is a requirement, getting your point across clearly is optional.

I don't know if I agree with the optional part. I actually believe that conciseness/simplicity follows clarity. They go hand in hand.

This French quote comes to mind. Makes me believe that what Pascal meant is that he didn't have enough time to reflect before writing the letter...

Ce que l'on conçoit bien s'énonce clairement, et les mots pour le dire arrivent aisément