It will be the death of the frame eventually, the rust has pitted the metal significantly – if you had it 'blasted' it would more than likely go through. You will proably get years out of it yet though.
fastindain – no no no! Body filler is not designed to go over open seams, it will crack and mositure will get in. 'Seam sealer' is what you need, it remains felxible and is designed for the job. The amount of cars I see where filler has been used over unwelded seams – shocking, they always rust out. Sadly for you, thelegsdontwork, the horse has laready bolted so to speak. http://www.peterflynnclassiccars.com
If you had some boutique type frame, then I guess ther would be some mileage in getting the frame blasted and repainting, sealing the horrible gusset gaps but…………….