In short, "infection parties" are an unnecessary risk when vaccines exist. Even if you do nothing else to protect your child and they experience a breakthrough case, they're a lot less likely to be hospitalized and potentially suffer lifelong effects if they've been vaccinated.
It's not so much "controversial" as it is an archaic idea. We've been there, done that, for chicken pox, measles, etc.
https://en.wikipedia.org/wiki/Pox_party
In short, "infection parties" are an unnecessary risk when vaccines exist. Even if you do nothing else to protect your child and they experience a breakthrough case, they're a lot less likely to be hospitalized and potentially suffer lifelong effects if they've been vaccinated.