Our company has a specific procedure where you can release the brakes after the aircraft is all closed up, tug attached, and you are waiting due to an ATC delay.
I have no problem with this... its is an organised, understood procedure with both pilot and engineer being on the "same page" knowing what will happen.
In these circumstances, you are ready to push, so there should be no people or vehicles around to be run over.
If crew are 'tricking' the ACARS system without ensuring the wheels are chocked first, then they're idiots. There is nothing unsafe about releasing the park brake... its only an issue when ground staff are not aware it is going to happen.
The REAL safety issue (if there is one) is having crew on OTP bonuses of any nature, and I would hope that this doesn't occur in the civilised world!