When I started work I produced little output, because I was a newbie and knew nothing. I was paid just a little over minimum wage - but I still produced, even if it was crappy code and routine tasks nobody else wanted to do.
Companies used to do this all the time - you start at the bottom, fresh out of school, and you have to learn.
But you are still f*ing paid.
If (as an experienced hire) you start a job somewhere enterprisey, the amount of bureaucracy and induction procedure you experience in your first week can mean that you won't be a productive employee for at least a week, if not two. You'll probably still be taking time from experienced employees for months. You still get paid.
In such a place, the first week of employment of an experienced hire is exactly the same as it is for an intern.
Yes, an employee takes just as long to get up to speed as an intern, but hopefully that investment is paid off by years of work afterwards.
For a two or three month internship, you might spend half their time in getting them up to speed. And for that, you get the same amount of work (best case), then they are gone.
Yes, internships can be a great way of recruiting, probably the reason most companies do them at all, but saying that the upramp is comparable to normal FTEs is ignoring the fact that they aren't long term.
My point (possibly not particularly well made) was not that you should take on inexperienced pretend employees, treat them like real employees for three months, then get rid of them; but that you should take on inexperienced real employees and treat them like real employees.
Three months is a pretty normal probationary period for a real employee, so you can just as easily get rid of one if it doesn't work out; as you can an intern.