I use one
Being slightly skinny of frame, the protector bulks me out and means the jacket is a better fit. As to comfort, I don't really notice it one it heats up and forms to my back.
Protection is a trickier question. They can help absorb impact. They can help spread an impact. They can also help against penetration and abrasion. How much/little will vary from design to design . What none can help against is torsion, crush or over-extension.
Compared to the foam in the jacket (most 'protectors' are just foam padding and offer no protection at all), a protector will normally extend from coccyx to shoulders, with a wrap around the kidneys and covering the shoulder blades (again, designs vary) but it's almost certainly an improvement on the jacket alone. Also, as the protector is independent of the jacket, it is much more likely to stay put in a crash an do it's job. If your jacket is loose, the protection it offers could shift and expose you to the full force of any impact.
Should you wear one? That really depends on your attitude to risk, how well it fits you and how well it sits with your other armour. No point in wearing it if all it does is distract you or hinders other protection.
If you are genuinely worried about your spine, consider one of the neck brace that are on the market. They can protect against torsion and over-extension of the neck.