[sanderjd]

I think you're wrong about the first part. It's probably right for newborns but not even older infants and it is definitely possible (even extremely common) for toddlers to get too little sleep if left to their own devices.

"Sleep when the baby sleeps" sounds good but is not really how sleep works; sleep quality is very sensitive to cycles and time of day (it's also a very personal thing, with big differences between individuals). It also doesn't work at all for working parents. And even in households where one parent doesn't work, the other one does, and it is important for both parents to be rested.

Of course you can pull this off, I'm not saying anybody is going to die or anything, I'm saying there are very real advantages to more predictable and consistent sleep. For us, we were very hesitant (or maybe just lazy) about sleep training, but our ~15 month old was noticeably happier after being able to sleep through the night.