This problem is part economics and part psychological.
If you make it too rare and access is sporadic, you will have people hord it and not spend them on troops.
If you make it too common, it ruins the value and purpose of an intentionally rare resource.
What you have to do is find a happy middle, and people have based careers off of attempting to solve this kind of problem (based on real life resources and products). If it is not too rare, then Ironicly more is spent, and keeps it from being too abundant.
Right now, I'm enjoying my surplus of DE, since hording and revenge attacking to recover losses (or gain a profit) and the occasional raid nets a good amount, has given me a max level King, and most of the Dark Barracks troops at max level. Now that I can't dump a huge amount into something anymore on a regular basis, I now feel like it is worth it to use Dark Barrack troops! Before, rarely touched them, since I didn't feel I'd get my 'money's worth' out of them back then. DE was just 'too awesome to use' before on expendable things (like troops), when it was really rare for me.

