A scam is a scam, no matter in what stage!
I do not want to make money from the misfortune of other people, even when I know the only reason they participate is because they are greedy.
That would make me no better than them!
MLM companies are not a scam in the first place. They always make their things legal by always featuring their products which they will declare that it is their main priority than the recruiting fee it rewards to their up line. They can get away with it as they have a product that they are saying it is what they are selling. The only way the company fails is when there are no more recruits but the up lines are still earning money through their bonuses. Also MLM companies that disappear quickly are the ones who have failed to market their company therefore not having enough income to share to the investors.
People often confuse MLM with a Ponzi scheme though as the two have similar traits, so we might as well treat this thread as referring to a Ponzi scheme.
For me it's all about the ethics of investing in it. Sure, it's a risky investment, but those who put money in early can actually earn quite a lot sometimes, so all that really matters is whether you think it's moral.
On the one hand, there's the fact that all of your earnings will be from people who joined too late and most of them will have lost their money, which some people might be uncomfortable with. However, if you don't refer anyone and support the scheme in that way, it might still be morally acceptable to invest in the scheme because you aren't making more people invest in it yourself. I might consider it, but I much prefer legitimate investments just because of the fear of putting my money with someone who most likely intends to scam me.