Interesting.
I like the concentration mechanic as-is, and most suggestions I've seen to get rid of it would overbalance the caster classes. This is the first house rule for allowing concentration on more than one spell that I might actually consider allowing in my games.
It was also proposed so that, say, a wizard could have up two effects to lock down two guys, but he's trading his turn to keep the second guy locked down.
We actually didn't go with the idea. The last 5e game I ran we capped at 9th level with a bard, a warlock, a paladin, a sorcerer, and a fighter who took a few levels of cleric. In the penultimate session they knew an army was marching on their town, because the wizard in charge of the army intended to perform a ritual there.
Full story, but not relevant to the main post
[sblock]A nation had its laws inscribed in a book artifact that prevented anyone outside the line of succession from ruling. I'd established Baphomet, as the demon lord of savagery, had the power that the presence of him or his unholy servants would cause text in nearby books to be unmade. In order to destroy the Book of Eight Lands and establish a magocracy, the wizard intended to summon Baphomet. The whole campaign had a 'book' theme, with demiplanes like out of Myst.[/sblock]
The PCs didn't need to defeat the army, just kill the wizard, without whom they couldn't do the ritual. So they devised
Operation Bloodtalon. They scouted the location the army would march and found a steep hill they would need to pass, then hid just out of sight. When the stone golem elephant - atop which the lead wizard and his erinyes bodyguard rode - tromped past, the party struck.
The cleric cast enhance ability (Strength) on the bard.
The paladin cast bless on the bard.
The warlock cast fly on the bard.
The sorcerer cast haste on the bard.
The bard cast greater invisibility on herself. She was a grappler build.
Next round, everyone readies an action to attack, except the bard. She flies at incredible speed over the hill, invisible, and grapples the wizard, then dashes back over the hill. Everyone attacks the wizard. He dies.
The army freaks out and tries to crest the hill. The sorcerer and warlock each grab some people and dimension door away, out of sight. And that's why concentration is still perfectly fine as is.