I think sim time is useful regardless. That's because in day-to-day flying you can only work within the parameters of normal commercial flight, whereas to really "hone" your skill, you need to keep exploring limits of the flight envelope.
You're not going to do approach to stalls or practice unusual attitude recovery with 400 souls on board.
But "free-play" is not training. For training to be effective you need to have clear objectives, a structure/curriculum, and measurements of areas to improve.
You don't want pilots developing bad habits from free-play that they will take to the aircraft. Or learn something wrong in the sim due to lack of fidelity and supervision, only to get surprised when the behavior of the real aircraft is not at all the same.
Airlines are better off training pilots as a crew, not just have individual pilots go at it in a basic FTD in their own time. Which brings me to another point... training should not be "voluntary" (unpaid).