He would use the Mercs obviously if his army was weaker than his opponent's or if he needed help defending his town or if he was doing some kind of raid (for the next sentences, count out the raid). If he had a whole bunch of slingers at his disposal and his enemy made all archers, there would be no need for Mercs. So why make a bonus against Mercs? No archers have bonuses (archers, not counter-archers) so why start with Mercs? Meh, that paragraph didn't get my point across.
What I'm trying to say is that Mercs are sort of a last resort thing, "sort of" being keywords there. If archers got a bonus versus Mercs (and you were pumping infantry) and your opponent was spamming Mercs, you could just switch over to archers and there goes too much gold down the drain for an unfair advantage to you. It would give every player an unfair advantage versus Mercs. They are supposed to save your "behind", and if you were killing an Eggy pretty badly with infantry, you could just instantly switch over to archers and kill the Mercs he spams like flies. It would be useless to spam Mercs if that was the case, ESPECIALLY versus an Attie who spams Turmas. That is one of the Merc's main functions, to assist your army, and archers having a bonus versus them would completely nullify that. Archers are, according to their In-game help, good against infantry. Mercs are infantry. While we're at it, why not add a bonus against ALL infantry? You can't just give all archers a bonus vs a certain type of infantry. You add a bonus versus Mercs, you add a bonus against ALL infantry.
He could make rax units, yes, but what happens once the raxes get destroyed/and or he runs too low on wood/food? Mercs. Oh but wait! Archers get a bonus versus them. Good-bye gold. Even if you weren't low on food/wood, Mercs are a nice little squad of guys to have defending your town. Regular archers (besides Gastraphetes) have no bonuses. Let's keep it that way.
Don't give things bonuses versus Mercs, but instead: increase production time, increase price, decrease stats, decrease life span, etc.
And anyway, if they DID add that bonus, Norse wouldn't have a way to counter Mercs. Well, TA I guess...
Meh...