Some people are paranoid that the chemicals used will mess with you pretty badly. Of course, research that points to that conclusion is often flawed or at the very least woefully biased in one way or another.
For example, saccharin was banned a long time ago because of a study where rats were fed a metric fuckton of saccharin (I don't remember the exact numbers, but it was a ludicrous amount). One of the rodents developed bladder cancer, and the shit was history. Even wikipedia leaves out the details and simply states that saccharin was linked to bladder cancer in rodents.
But yeah, there's a whole host of silly tinfoil hats out there who claim that aspartame, ace K, and sucralose are carcinogenic and blah blah blah blah. All I know is that I don't give half of a fuck.