The biggest question with regards to frame mounting is the integrety of your existing body....add some rust in and a frame mount is a nice safety factor.
I used sandwhich plates for my main and front hoop and the rear stringer is a custom mount. The top and bottom plates should be different sizes so as to not create a shear point in the sheet metal.
IMO, it is minimal expense to add in the frame tie in's and with the minimal weight gain it's a worthwhile safety point.
Do you need it? 99% of wheelers only roll on their side or once over, and 95% of those are so slow you feel it's taking forever. The overbuild factor is for that one time you roll hard, when your running a ridge, and the bottom is 4-5 rolls away. I've done those and have come close to rolling...I was not scared cause I trusted my cage design and mounts.
I like my cages to be removable, for future bodywork needs, cleaning purposes, or should you change your cage design and want to remove your existing one.