Actually you're right, it would get rid of the worst of the lemon dropping, which is when they find excuses to dump cancer patients with the least ability to retaliate (least family, least social media, whatever).

The example that was at the top of my mind when I made that claim was the cherry picking, because at this point in my life I'm a cherry, so that's what I have experience with. It's the situation where insurance companies all provide first year discounts, encouraging companies to switch every year, and then make getting set up a hassle so that relatively healthy people forego interaction with the medical system entirely.

Reinsurance wouldn't fix that problem, but that problem is much less pressing than the "dump cancer patients" problem.