Personally, I don't like them on anyone who is actually planning on going into the water; they come off too easily, and IMO that's a safety hazard in open water. (And I'll admit it, I'm prejudiced about people who take up space on a beach and never set foot in the water -- they could soak up the sun anywhere, so they are making the beach more crowded for no good reason. And yes, I know that's an unpopular opinion, but there it is.)
As to suits that don't fit, well, that can happen with any suit, and it's always wrong. At the pool I frequent in summer, there is a lady who comes with her DH to swim laps. I'd guess she's about 70, very thin, and she's there every day. Unfortunately, so is her suit, and it's past it's prime -- WAY past it's prime. The suit is a plain tank that I'm sure was very modest when she bought it, but at a guess, that was probably about 15 years ago. I think that it may have originally been blue or purple, but now it is a very faded grey, and has worn thin in several spots, which is bad enough. The worst, however, is that the fabric has completely lost its stretch. The armholes and neckline now hang VERY low, and the rear end sags halfway to her knees. The thing sways in the breeze when she walks, and essentially, everything shows. Honestly, I'm waiting for the day when she hauls herself up the ladder and leaves it behind -- it's THAT loose on her. (Honestly, I've been SO tempted to leave $40 in her gym bag with a note to ask her to please go and buy herself a new tank suit.)