|There are some weird controllers out there (myself included) that have this wacky notion that going down and slowing down isn't all that easy (wonder why!), so an unrestricted descent will help with that aspect.
I wouldn't look at it as "efficiency" as such, more that controllers appreciate that YOU are flying the plane and will have your own preference for how you want to "get down". 90% of pilots understand that once you are within 25 miles of YYZ on arrival you are being sequenced for the approach, and need to be lower anyway, especially if we are running a hi/low in IFR conditions.
We care less about the restrictions than we do about what altitude/speed you are at when you are in the downwind. I would rather have it nailed down by a certain point (level usually) than have you going through the various restrictions (say VERKO/MAROD on the 24s). Easier to control that way....hopefully easier to fly.
When it is busy, the restrictions become a bit more important as the speeds on the STAR help with the arrival sequencing.