Some organizations openly set out the first few weeks or the first month or two as a probationary period for new hires, but you're right about the cost effectiveness to some extent. A six month recruiting process might cost a few hours or days every so often from a few employees, but won't add up to even a single month's pay for a bad employee. Add in a cultural practice of risk-averse hiring (don't hire any B or C players) tilts the scales even further.

That said, I can imagine some cases where opportunity cost and being more concerned about getting good employees than immediately weeding out potentially bad employees would lend one to the probationary thing.

Amen. Someone in my orgzn was let go very recently. I'm not in mgmt so only had a peripheral view of what transpired, but the orgzn I'm in is very loose and generally hires strong candidates with very high success rate. We don't have a lot of time to spend monitoring performance closely; we expect people to work independently and deliver in a non-sweatshop environment. Once a problematic situation is recognized, it sucks up a lot of time, energy, and karma (heavy HR involvement, PIP programs set up, tracked, etc.), and of course we have tasks which the individual was expected to be completing; now not so much. Also, the months spent bringing up to speed someone who will now never contribute to the team, time and teammate energy which could have been expended on a better candidate. The net cost has been huge. I remember there were some doubts about this individual at interview time (and our interview process is nowhere near as severe as what is described to take place at Google, etc.), and these were ignored (there were "extenuating circumstanced"). I'm reminded of a quote from Ronin "Whenever there is any doubt, there is no doubt." Even though from a fictional source, it's good advice. Also: "no exceptions".