People being good at anything else (let's say, accounting) is not correlated with safe/smart computer use habits. If you can prevent a lot of turmoil by not letting them install arbitrary programs and still keep them happy and working well on their main role, why get rid of an otherwise good employee?

Very good question!

I think it's useful to distinguish between types of trust. You might trust a mill operator with $5 million worth of automated equipment that has a well-documented UI backed with years of training and industry support. However, you might not trust that same operator with unfettered control of a $2000 general-purpose networked computing device that can silently leak company secrets and offer a foothold to attackers inside your firewall _even if the operator does nothing obviously wrong_.