Cliff
As someone who has fitted a rear toolpost, mostly used for parting off, I can confidently say they are worth the effort. I fitted a long T slotted cross slide mainly so I could fit a rear toolpost and it has been worth every penny.
On my elderly lathe, before I fitted the rear toolpost I often broke parting off tools and I had to reduce speed to part off successfully.
Since fitting a rear toolpost I can part off at the same speed as I am using to turn a part and 'touch wood' I haven't broken a parting off tool in years. The theory is right, dig ins reduced dramatically, parting off is easier..
John