Bathtub and Shower Pan fixing flanges ???

I am researching the pros and cons in regard to the installation of bathtubs and shower pans with fixing flanges compared to the installation of bathtubs and shower pans without fixing flanges. Is joint movement between the wall and bathtub/shower pan fixing flange ever a problem? Is there any benefit isolating the wall from the bathtub and shower pan in installing a flexible seal between?
Thanks, Gerard
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