Don't Knock It Until You Try It? Nah.
Personal experience is "sample size of one". It is noise. It subject to sunk cost fallacy, subjective validation, self-superiority bias, confirmation bias, and all the other cognitive biases. Mankind invented science and scientific process to counteract such biases. Personal observation is subjective, and therefore biased information. Advocating one to "try it" simply proves nothing.
Yet MLMers love to fly this particular argument. They value personal experience over all others, the exactly opposite of scientific process trying to filter out bad data. It is basically fully faith-based.
I was skeptical until I tried it
A true skeptic would know NOT to try it due to all the reason discussed above. The proper way to evaluate something is through scientific and statistical process from a large sample set, not through a single subjective personal experience.
Falling for a dare / lure to "try it" just makes you gullible, not skeptical. Yet MLMers selling nutritional supplements or unproven "treatments" love to fly this particular argument. (also see "What's the harm" below)
I know it works, because it worked for me
So somehow, you're God, and what you experienced is the universal truth for everybody, eh? It's just your subjective experience, based on your circumstances at the time, and based on all your PRIOR memory and experiences. If any one else had different life history, experienced the same thing at a different time, under different circumstances, or any combination of such, the experience will be different.
What you experienced is only good for yourself. It is not a data point. It is anecdote.
What's the harm?
The harm is people will pay for the bogus stuff rather than stuff that really works.
For income opportunities, they would at best, wasted months (or years) "treading water" spending time for no appreciable result, or at worst, losing thousands or tens of thousands of dollars in the process, as well as alienating all friends and family. There are occasional exceptions that do make a living at it, but that's the exception, not the rule.
For nutritional supplements and such, the stakes are much higher. One winces at the stories about gullible people stopping their medicine because they believe their nutritional supplement will cure them. At least one caused permanent brain damage in his daughter using nutritional supplement as potential cure.
"I know what I saw"
Nope, you don't. You interpreted what you saw to suit your narrative. That's why there's the scientific method to discount anecdotes, and rely on repeatable experiments to gather reliable data.
How about magic? Magic tricks are obviously "impossible" on the surface, yet it's done. When you see magic, do you believe magicians can levitate people, cut people in half and put them back together, and so on? Of course not. You know such things are impossible and it's merely an illusion. So why can't you accept that OTHER things you see may be an illusion as well? A fraud, an act done for your benefit? Ponzi schemes pay people, often in big ceremonies with the fancy show checks, to "prove" to the sheeple that it really pays. But it's a facade depending on more people paying into the scheme. The net winners getting paid creates the illusion that everyone will be paid, when it's clearly impossible. People who saw others getting paid (or are paid themselves, i.e. "proof is money in my bank account") are quite fond of this argument. Yet few if any of the victims realized that Paul Burks of the ZeekRewards ponzi scheme, and recently found guilty of 4 counts, was a trained magician.
A scam bomb detector maker called Quadro Corp scammed dozens of police officers into recommending purchases even though one was taken apart by Sandia National Labs and found to contain NO electronic or chemical sensors inside. Yet none of the police officers scammed by bogus detection tests were willing to admit in court they had been duped, and instead, insisted that they saw successful "trials" (which were clearly rigged, as the results are impossible). The jury was forced to declare the company heads "not guilty" of fraud despite FBI warning the police across the country not to buy the fraud. The police officers "knew" what they saw too, even though it's impossible.
You may have seen what you saw, but you could have misinterpreted it, or have been deceived into making a particular conclusion that's not the truth.
"You are close minded"
MLMers are quite fond of throwing this out, when they ran out of arguments. They then accuse you of unwilling to see their side of things, despite them giving flawed, non-sensical evidence, or often, no evidence at all.
As Carl Sagan said, "Extraordinary claim requires extraordinary evidence." Open-mindedness is willingness to admit one is wrong when presented with evidence to the contrary, not merely willingness to try nonsense. If you provide evidence, I'll evaluate your evidence and if necessary, reconsider my position. If not, why should I even treat your position seriously?
"I'll try anything" attitude is one of an adventure seeker, not of a skeptic.
In not understanding the definition, the accuser is the real closed-minded person, yet accusing others of close-mindedness. Ironic, isn't it?
When you see these arguments being bandied about, it's time to avoid whatever is being advertised. They have no proper arguments, no scientific evidence to back it up, just stories by unreliable narrators.