It looks like that because the teams that use it in unbalanced leagues are sharks vs minnows. Against an equal opponent the "wingbacks" are fullbacks.
And nothing is new under the sun, but its shiny new because its in fashion presently.
And why is this bad, other than being too trendy for you?
That's one of the advantages. It has the flexibility to be one of the most attacking formations and one of the most defensive formations.
I used to think the problem with the 3-man backline is that it couldn't handle a good 3-man front well, as the opposing wingers would get behind your midfield and then stretch your 3-man defense too thin across the width of the pitch. But we have a pretty good 3-man front, and Toronto has had zero issues with us.