No independant doctor is going to say, nah you're not concussed mate, get back on the field.
It won't be the likes of Warnock teams and the Baggies expoliting this - their benches aren't really deep enough. If anybody 'makes the most' of the rule it will be the likes of Liverpool and City.
Overall I think it's a sensible idea. I just hope it doesn't allow 5 subs to sneak in by the back door.