I don't think sticking a bunch of Ithilmar-armored melee types in a troop slot has fucked our ability to use Ithilmar on Elites for the rest of our army.
We say "this squad is Elite, this one a Troop, this one a Heavy Support, this one a Fast Attack" when we design the troop, not when we design the gear that goes in it, and it's not like we lose the ability to make
different squads with that gear.
So, like, if we made a Troop squad filled with several light tanks, it'd be a Troop squad whenever we raised it, and it'd stay a Troop even if we later put it into a Warhost that wasn't intended to be dedicated heavy metal Warhost.
The cost is that when we design a Troop, time spent designing a Troop that only fits into one type of Warhost that we won't have that many of is time not spent designing troops that almost every Warhost is going to be using day in and day out.
The actual difference between the troop types in practice, outside of "whatever we stick in them"... we know it impacts how many we can include in a detachment, with Troop and Fast Attack being able to bring more and Special being able to bring less. There's probably other impacts what we're not aware of, either from Peaceful or just from inexperience, e.g. Elites possibly being made up of more-skilled soldiers.
Also, our commanders (who we do not get to micromanage) are presumably going to use troops by their designation, so Fast Attack get the orders to make flanking attacks, Troops get the orders to hold ground, Heavy Support get the orders to back up Troops who've run into trouble, that kind of thing.
I don't agree with
@Alratan that everything in a detachment needs to have the same speed; yes, they'll collectively move at the speed of the slowest element, but that's for strategic movement; tactically having units in your forces that can quickly respond to things faster than the rest of your forces is basically what Fast Attack is
for, and we don't want tactical situations getting kicked up to the guy in charge of the entire Warhost if it's a detachment-scale problem that they only can't deal with because we specifically denied them the proper tools.
What we
do need to pay close attention to is the slowest speeds in the detachment, and consider the impact of adding a unit slower than that very carefully.