FormerlyHemlock
Hero
I'd say the kneeling guy would still be granting advantage vs third party attackers.
I do think it should be possible to 'Pin' a grappled foe in 5e, maybe with a second grapple roll (you have advtg vs target, target has disad vs you, third parties have advtg vs you) but there are no RAW rules for it. And clearly you can't be standing up while holding on to grappled target with your hand.
There are RAW rules for it under the Grappler feat but they are goofy: both you and your target are Restrained.
While we're on the subject of interesting combos, here's one: throw a net on someone in a Stinking Cloud, and then get out of the cloud. Nets restrain the enemy (speed = 0) until they spend their action to break free (DC 10 Str), or attack the net and deplete its HP. (AC 10, requires 5 points of damage or more from slashing weapons.) Stinking Cloud creates heavy obscurement and causes any creature that starts its turn within the cloud to Con save or lose its action that turn (but not its movement). You see the synergy?
Stinking Cloud obscures vision and grants the usual (advantage + disadvantage = nothing) to cancel out the disadvantage from using nets. The net, if it hits, will prevent the enemy from moving out of the cloud. The Stinking Cloud has a high chance of preventing the enemy from breaking out of the net. Net keeps the enemy immobilized so that you can, if you want, enter the Stinking Cloud every turn (you're not affected because you didn't start your turn inside), attack the restrained enemy, and leave the cloud again (no opportunity attack because the enemy can't see you).
A high-level Valor Bard with War Magic can cast Stinking Cloud + bonus action throw net to pull this combo off entirely by himself... but if your party knows how to cooperate with each other, you can pull it off much earlier. Potentially using poison-immune Skeletons to throw multiple nets per round, if you want.
Throwing a net on a prone, grappled opponent with your free hand is another way to mess up his day even more, to prevent him from getting lucky and breaking your grapple. Now he has to break your grapple and cut your net in the same round, or else it's useless and you'll just re-grapple him next turn. Also, it restrains him so that your ranged allies are no longer at disadvantage to attack him.
Finally, don't forget that dropping prone can sometimes be advantageous. For example, it's one way to counter Sharpshooter. A wizard standing up is relatively easy meat for a Sharpshooter. A wizard lying on his face launching Fireballs is significantly harder to kill. For a 12th level Dex 20 longbow archer fighter vs. an AC 15 (+Shield) wizard, it's the difference between inflicting 36 points of damage per round via eyeshots (21 points of damage if the wizard Shields), vs. inflicting only 21 points of damage (eyeshots don't change the average) or only 10 points of damage if the wizard Shields. It also reduces the rate at which the wizard has to burn Shields.