Bodhbh
Guru
Mostly it's BSOs that have them and as such they are useful. Basically because the "Sunshine Fastracer" freewheel and the "Cheng-Hing Foo-Yung" chain don't work all that well with the "Honlable Star Index Gear" and the chain does come off the inner sprocket, when the limit screws shake loose.
A properly set up bike with at least basically decent components shouldn't need one. Cheapo gears - yes, the disc saves even more aggro when it all goes wrong.
That said, it didn't do much to help my OH's BSO over the weekend. It took about 5 yards for the deraileur to end up in the spokes.
Instead of removing it, OP should generate some talk and fix a matching one at the front.