I don't know if it would be a good decision of Merry to appear with the Rolling Stones nowadays. On Yutube you can find a great live version of her performing Gimme shelter, and it would probably be worthwhile seeing her if she performed with that crack band. But she could never repeat the spectacular performance that is captured on LET IT BLEED so fans would be disappointed.
She's great in her own right but she should leave that classic moment she shared with Mick Jagger as an outstanding event.