Yes of course, electing to continue on the Sickness Benefit is particularly good value. Unlike private insurance, it stays with you for as long as you contribute, and your contribution remains stable with advancing age. One other aspect is that my country (Belgium) is negotiating with Thailand on a bilateral Social Security Agreement, that would allow people in either scheme to benefit from the other. This is a bit of a long shot, but if and when ratified, it would enable me to get back under Social Security "using the back door", so to speak.
They wanted my Yellow Book, they did not ask if I was a PR or anything. I did not inquire about what would happen if I were a PR, but I would assume that if you are one, you have a Blue TB and a Pink Card, you are all set. In practical terms, getting Social Security to accept one under Section 39 is quite straightforward, but they probably are making it harder than it should be by demanding quite superfluous documents, just as additional roadblocks.
There was no mention of a refund or anything, but I have not been in the SS for very long. I had known that Section 39 was available for a long time, I simply arranged for a visit to the SS office with a Thai speaking staff from my former employer, and my request was turned down. it will remain stalled until I can produce the Yellow TB and Pink ID they want, and then it will resume and eventually go through. Either that or I will reintegrate SS by way of Section 33 and "classic" employment.