I realize thousands have been stuck with grout and held up fine but I use thinset to stick soap dishes and towel bars, alot better sticking power than grout. I've seen too many fall out that was bonded with grout. I'll never forget the job I subbed out, the soap dish fell off and chipped the bathtub, you got it, he stuck it with grout.
I did have one fall off that was stuck with thinset, the painter stood on it and it pulled the mud and all off the lath, kinda like the picher shows. Grout won't hold that good.