Mercenaries got an indirect, but huge change from mk II to mk III, buffs requiring a faction friendly unit. This leads to the question, “why take them”? The easiest way to explain this is to take a quick look at two very comparable units, Temple Flameguard and Steelhead Halberdiers. Both of these units cost the same number of points, fulfill the same role on the battlefield (a jamming/tarpit unit), nearly same stats and have nearly the same rules. The main difference, where Flameguard have the Shield Wall order, Steelheads have Powerful Charge (Steelheads hit a little harder than Flameguard, but Flameguard are slightly more accurate). The way these two units actually play on the battlefield is different despite their roles being identical. The Halberdiers want to charge their tarpit target and they threaten 11″ while doing so (and charging mitigates their lower accuracy for a turn). Flameguard tend to be slower since they want to stay in Shield Wall for the most part.
So we have two units that are nearly identical, but with the change to buffs, why would I ever take the Steelheads? Because I can’t put Defender’s Ward on two units simultaneously. Buffs are selfish, they can only be put on one unit/model per use and that unit/model can only ever have one buff. For Protectorate, there are a number of casters that have one defensive upkeep that they’ll want on their jamming unit, one offensive upkeep that gets cycled between units/models that are doing work and that’s it. This means that often, some units on the table will not get buffs until the game is hopelessly lost or won (#winmore_optional). With this consideration of one upkeep per faction friendly unit, there is some merit to running the Steelheads with the Flameguard as the first wave tarpit (since they are almost sitting on victim stats…). The Steelheads race down the board and spread out, trying to catch as many things as possible (14″ run to engage isn’t shabby at all!). The buffed Flameguard then follow-up, giving the things that the Steelheads caught something much stiffer to chew on (15/19 with Defender’s Ward isn’t shabby at all for such a cheap unit). Now all that is left to do is to figure out how to kill the things that have been caught!