Ah, I see. Well in that case I think it limits the fun of prestige troops. Let me give you my two cents.
Yes, it's nice to get free order troops from your chapters, but given that I now know prestige is limited, it makes spending it much less attractive. Seems to make sergeant troops especially a fairly bad deal. Why would I want to spend what seems to be the only limited and precious resource in the game on, say, griffon retainers, when I could easily pick up some better cavalry for free? Or dawn wyverns when I can get huscarls or men at arms from commoners? That's a real shame because those troops are cool. Also it encourages hoarding prestige until you bottom out their price due to high honor, which takes a long time and, again, takes the fun out of hiring order troops as they become available to help your empire.
This prestige mechanic also makes little sense generally. Taking renown first, you have this lord who is annihilating everyone, thus gaining renown. After a while, people get used to him winning everything and so the victories are not so impressive (diminishing renown returns) and if he goes quiet for a while it really affects his reputation (renown decay). That makes sense.
Now to prestige. This lord, while he is destroying everyone has attracted a great number of elite troops, and sought them out himself (spending prestige). Now he has reached the height of his renown. There is no earthly way this guy can get any more awesomely renowned. But all of a sudden no more elite troops want to join him, even though he keeps winning every single battle, simply because he is not getting any more renowned than he already is. If he's lost his elite troops already, or whenever he does, which is inevitable, he will never be able to directly get any more no matter how much destruction he wreaks, because he's not going to surpass his renown high point. So, even though he's the most awesome powerhouse of a lord in the universe and continues to own everyone, he has to rely on the very slow chapter houses for order troops. This makes no sense.