Training people is expensive in both cash for the business and the time of those around them. Hiring correctly once would make my life a lot easier.
I agree that training people is expensive - I'm just not convinced that any other system than the probationary one works. That is to say, there's sufficient cases of people getting past whatever screening plan the companies have and yet cannot do the job. Depending on the company, once you're permanent, it can be very hard and every expensive to fire you - especially in some countries.
I'm not suggesting that you should take anyone off the street and give them a probationary period. I'm saying if your position needs a skills assessment, I don't think there's a functional one other than a few months of actually doing the job. Too many other systems are easily gamed, or are easily set up to fail people inappropriately too.