Ah hrm, thank you for fixing my terminology.
I'm on the operations and architecture side, somewhat different than writing say cutting edge algorithms or what not... but ultimately we're talking people: someone not having the professional conduct to not reveal sensitive information is the same whether they work for 1 company for dozens at the same time: I would stand by my assertion there's no additional risk either way, either the employee is trustworthy or they are not and your case of someone's leaving the company to go build a competitor, that is a single individual's decision which doesn't reflect upon anyone else individually.
'92 was when IBM had their first layoff and it was a watershed moment in the industry: one of the last major loyalty to employees companies in America if not the last.
After that point as one of my buddies opined in the Austin newspaper: loyalty between company and employees is dead, now employees are loyal to each other. Funny how the C suite wasn't too happy with that statement hahaha.
The problem is by the time you find out your employee is not trustworthy, you could have lost IP worth many millions of dollars. And a lot of people don’t even realize they’ve revealed critical information even when they’re trying not to (hence calls by analysts, fund managers, investors, press, etc to random employees). Seems to me only prudent to have a moonlighting policy.