I use one just in case better be safe than to be sorry right. You can get cheaper ones fro Divoza it's a website. I ordered nearly al my tack from there.
Yes, and although it can get a little pricey it only helps.
I think you should have one because of extra comfort and support.?
I find it better to have extra comfort because just a regular saddle pad does not have much padding. Even when your saddle is fitted to your horse it's best to have extra comfort
Better to be safe than sorry, but if you have your saddle fitted regularly you shouldn't need one unless it's a gel pad for shock absorbency :)