There are (according to the BMI[1] (German interior ministry)) only two reason to have multiple passports:
The first being the one you stated and the second being multiple (visa requiring) travels in a short enough timeframe so that your passports would have to be lying at the foreign countries embassies/consulates at the same time.
Seems like motivation enforcement is quite strict, as they do state that you can't even get two passports for the "enemy visa" reason when the visits are just planned, but you can't prove it yet (palne tickets etc.) you won't get it, and you'll have to go to the German embassy after you can prove it...