While it certainly is true that you get a half stack or so of units when successfully demanding a city in diplo, these units are always producable in the settlement in question. I once took Saragossa from the Spanish by force (crusade, to be exact - they were excommed), after annihilating 2000 troops in the field. They sent a diplomat and begged for a ceasefire. I OKed that, on the condition that they give me Pamplona and Valencia for free. They were so awed by my military that they agreed, and I got two settlements half full of Mailed Knights! Both weren't able to produce anything else besides basic militia/levy spear. While the Spanish reformed a largish army and brought over their assault troops from northern Africa, where they had been whacking the Moors. Suddenly I faced an imminent attack on my shiny new prizes, having nothing but lots and lots of Mailed Knights!
I had to bring in reinforcements from France, and when they got there, I began disbanding the MK - they simply were no use to me whatsoever and were a drain on my budget.