I agree, the ARP Quartet sounded terrible, especially the organ. But there have been a few string synthesizers that sounded quite warm and beautiful, such as the ARP Omni. I had an Elka Rhapsody 490, and even that sounded beautiful when used judiciously.
I think what sets these string synthesizers apart from a Prophet '08 emulation is the paraphony, the built-in chorus, and the ability to combine sometimes three octaves of strings at once. You just can't do that on a single P'08/Rev2. But if you could, it still wouldn't be the exact same sound, certainly not regarding those wonderfully dreamy paraphonic attacks and releases.
There's a very distinct effect these instruments have when they're part of an ensemble. I admit, alone they don't sound too impressive - kind of buzzy. But processed or in an ensemble something magical happens.
There's also much to be said for having a dedicated string synthesizer.
Here are three example of serene string machine music. I've always liked these: