IIRC, you have to make a save if you hold your breath too long. If you cough, no save is needed. So, a character can hold his breath several rounds (depends on CON), cough a round (resetting the "timer" to 0), and again hold his breath...
I don't have MotW at work, but I encountered the spell as a DM and it is not that killer spell as most people think. It is good against spellcasters (when they have no silent dispel at hand), but that's also true for Silence (which is in level 2 and allows no save if used properly / tricky!).
Miasma won't help in battle against non-spellcasting enemies because it takes a lot of time until it becomes effective. As a fire and flee spell, it won't work very good either: the affected creature can simply wait until the spell expires (hold breath, cough after a minute, hold breath....).