It’s certainly a matter of economy.
Fortunately, this is already possible. For instance, my swamp things’ lower abasyssals: (http://kingsofwarvince.business.blog/2018/12/03/1500-points-of-abyssals-a-2017-project/) are actually Mantic Ghouls. They work fine as either.
Most of my humans feature in several armies. I even wrote a blog post about this!