Having it tied to only recruitment is silly. Recruit a frontline of mercs, then get your cav and artillery to max professionalism. And everyone saw it coming too...
This will leave almost no professionalism on almost any nation that can afford mercs.
I see where you're coming from, but merc spams by the AI already ruin their AP. They struggle to keep it up as it is. Reinforcing will just make it worse.
It will put players that use mercs as combat shields on equal ground with the AI. The AI doesn't know that it should recruit infantry mercs and drill cav/cannons. Plus where are these magical reinforcements coming from? If I combine two 500-man merc units and recruit another merc unit I lose AP but simply letting them reinforce I don't? Hurr durr design right there.
It will put players that use mercs as combat shields on equal ground with the AI. The AI doesn't know that it should recruit infantry mercs and drill cav/cannons. Plus where are these magical reinforcements coming from? If I combine two 500-man merc units and recruit another merc unit I lose AP but simply letting them reinforce I don't? Hurr durr design right there.