I prefer my native troops - even the poorer ones - to learn from experience. Using the Mercs in the manner you described might save me some troops but certainly cost me a chevron or 3. Even those cheap native levy troops start to become pretty effective once they've gotten into the silver chevrons.

I rarely hire Mercs unless I expect an immenent attack and don't have enough native troops nearby OR if I overextend myself a bit and need some extra troops for garrison duty.