Nerva,
Another reason for trying to get your MRs upgraded to kingdom-tiersis that when your empire growls large enough that you *have* to start handing out kingdom vice-royalties to vassals, any duchy-tier MR within a de jure kingdom that you give to a vassal becomes a vassal of the new viceroy, rather than you direct vassal. However, if the MR already holds a kingdom title, it will remain your direct vassal.
At the same time, in such a situation, if the empire's laws allow vassals to attack one another, then eventually said viceroy will launch a claim war against the MR ... and destroy it when he conquers it. So, this requires some fore-thought.
Also, in my experience, when a MR based on a titular duchy title gets a kingdom title, they destroy the titular duchy title. I've not seen a MR based on a de jure duchy title destroy the duchy title after acquiring a kingdom title.
When I play as BYZ (generally from the 769 start), I like to holy war for Famagusta (Cyprus) and set-up a spare son there as a MR. Cyprus is just a 2 county duchy, so there isn't a great loss of feudal vassals. Plus, there are a lot of counties in the immediate sea-zones (allowing for the ME to have more lucrative trade centers). And (unless it's a feature of the CK2+ mod I use), Cyprus has a kingdom title attached to it.