The question Nix linked is related, but I would argue that the new Vassal Limits that came during Charlemange do change the scenario a little. I had a Scandinavian Empire game that was into the 1300s when the patch dropped. In that game I had actually started handing out King titles to relatives just because it was giving me headaches trying to keep up with the 30-odd Dukes I had by that point (having conquered much of Germany and Lithuania as well as De Jure Scandinavia). Had I not already started handing out King titles I think I would have done been forced to do so just to keep under the Vassal Limit.
King Vassal Pros:
It reduces the amount of people you need to keep happy with you.
(Bribes and Honorary Titles get better mileage)
One King Vassal is much easier on your vassal limit than the half
dozen Dukes he replaced.
Troop management advantages. When an entire kingdom's liege levy
forms up in one place it's a lot less work gathering all your armies
together. Also, a Kingdom worth of Liege Levy can often handle minor rebellions by itself.
King Vassals are powerful enough that, early on, you NEED to keep
If a King Vassal gains enough personal power his opinion of you is almost irrelevant, he WILL join Independence factions. I had a King of Denmark and Skotland as a vassal, with his opinion at 70 he was still running a faction to secede, and holding around 70% power by himself.
When you first become an Emperor, I would argue against creating King Vassals. In the early days of an Empire a Kingdom holds a sizable share of your levies. As your Empire expands beyond De Jure territory and includes another Kingdom or two worth of territory a single Kingdom becomes less significant. At this point I would consider handing out the smaller Kingdom titles to Dynasty members without claims on the Empire. Of course, standard De Jure issues apply. Avoid handing out Kingdoms you hold territory within. -25 Desires Duchy of X penalties stack up fast.