What's the harm (logical fallacy)

Those kind friends who suggest to a person suffering from a tedious complaint, that he "Had better try Homoeopathy," are apt to enforce their suggestion by adding, that "at any rate it can do no harm." This may or may not be true as regards the individual. But it always does very great harm to the community to encourage ignorance, error, or deception in a profession which deals with the life and health of our fellow-creatures.

What's the harm is an all too common emotional appeal argument employed by cranks and woo peddlers (particularly alternative medicine and New Age woomeisters) used to defend the usage of their particular bullshit. The line of reasoning is that because there is nothing inherently harmful with something, that it should be tried. It may be true that a harmless thing might be beneficial, but its harmlessness is not standalone proof; further reasoning or evidence is required. When used by woo peddlers and their advocates, it's often a poor escape hatch argument.

It is commonly used after the rest of their BS claims have been thoroughly refuted. As such, it is a form of escape hatch.

The basic argument
[Insert woo/crank idea here] hasn't caused any harm. Therefore, [Insert woo/crank idea here] should be tried.

While it's not always necessarily a bad line of reasoning, as there are plenty of activities that are inherently safe and enjoyable, such as listening to music, trying a new sport (which, by the way, is not harmless, as anyone who has been injured while doing sport can attest to), painting, etc., when used by woo peddlers, it's a means to defend their practices. Unfortunately for them, there actually are inherent harms associated with their practices.

Waste of resources
You are essentially losing time and money on something that is ineffective, as opposed to spending said resources on an actual solution. This benefits no one but the crank who tricked you, who will happily take your cash and run (or stick around for more, if you're of a mind).

Supports ignorance
No one benefits from ignorance, and in many cases it can be downright dangerous. Spreading misinformation to desperate people who actually need a cure is both dangerous and unethical, as you may very well be endangering their lives.

Deprives an individual of an actual cure/solution
This is where the real harm comes into play: people trying unconventional solutions often avoid actual solutions to their problem. For example, when Suzanne Somers had a breast tumor removed, her doctors recommended she undergo chemotherapy as a precaution, in case there were metastatic cancer cells they missed. She refused, because she was taking fermented mistletoe extract "instead". Luckily, she remained cancer-free, but it could have gone the other way.

Telling a cancer patient to try (say) cannabis oils, as opposed to evidence based medicine, and attempting to defend the practice with 'what's the harm' is evil and fallacious, as you actually are endangering another human being at that point. Even if they are using woo treatments in tandem with legitimate medicine, you are still wasting resources, as per point #1.

Criminal justice cases
Especially in high-profile crime cases, when a suspect gets taken into custody early, published as the likely offender, and ends up receiving disproportionate focus from the media, people sometimes ask "Well, what's the harm?" If they were somehow linked to the crime or were in the exact area at the time (and especially if they seem really shady), don't they almost deserve to have their faces plastered in the news and their role in the entire mess focused on?

The problem with this reasoning is simple. When investigators and the general public get stuck on a suspect early in an investigation who actually turns out to be less and less likely to really be the culprit (or lone culprit) as time moves on, the actual offender is left with far less heat. While it may be comforting to watch the news and see numerous suspects lined up right away, and while the police may score a PR victory for "demonstrating" to the public that they're on top of things, the fact remains that less time and energy gets devoted to finding the real offender when the bandwagon takes over.

And the sentencing of an innocent man does more than just that. It also stops the police from looking. They caught the guy, right? As a result, for every trial that turns out to be caught up with the wrong person, a guilty man walks unbothered in the streets, perhaps never having justice served to him as a result of the delay and confusion, possibly giving him time to strike again. Sentencing the wrong person doesn't just ruin the life of an innocent person; on top of that, it essentially gives a pardon to the real criminal.

Donald Trump
In 2016, Donald Trump used the equivalent of "what's the harm" in an attempt to sway African American voters:

Look at how much African American communities are suffering from Democratic control. To those I say the following: What do you have to lose by trying something new like Trump? What do you have to lose? You live in your poverty, your schools are no good. You have no jobs. Fifty-eight percent of your youth is unemployed. What the hell do you have to lose?