[spoiler=the important]Marches
Marches are a new type of subject that can be created from existing vassals. By designating your vassal as a March, you are giving that vassal greatly expanded autonomy in exchange for greater military service. A March does not pay taxes to its Overlord and cannot be diplomatically annexed. However, they get a 25% bonus to manpower, a 30% bonus to force limits and have 20% better fort defense, making them useful as military buffers against enemy states, or when you simply need additional soldiers more than you need the income from those territories. March status can be retracted, but doing so results in a stability hit and a very large opinion penalty with the vassal whose autonomy you just revoked.
O...kay... There's no reason to make marches, then. Your vassal simply gets free manpower and doesn't have to pay you money [one of the main ways I limit vassal power is limiting their income], yet you can't ever diplo-annex or force them to allow you to control their soldiers [leaving it up to the simply god-awful war AI to let them control their soldiers for you]. Why would I make a march? There's literally 0 benefits to the real life player in this situation. Just gives the shitty diplomatic AI another reason to not be a good vassal despite you having created the entirety of their nation.
Expanding on this, WHY would I ever need a march? One situation where a totally useless vassal that you can't directly control the military of being more useful than a vassal who pays you income and can be annexed. I can't think of it. The only possibility of marches being useful is if they make it so your vassal has +200 relations. Otherwise the AI simply will break off as soon as it can with the free soldiers and money it [thinks] it has.
By the time you're hand-crafting marches to 'defend your empire from outside forces' you've already blobbed to the point that your own manpower is likely plenty, plenty enough. The only time a march would be useful is in early blobbing [if the AI was useful at warring, which it's not], but the thing is.. after you blob.. you don't need to make little vassals to give you 1k or more troops than another vassal type. Especially if it just becomes a roadblock to expansion forever.