My experience has been that priority handling is directly linked to the airline and counutry where performance/quality of service is valued. For example, if destination airport is in Canada and airline is Air Canada, priority means nothing. Similar experience in USA on AC flights. If unloading is airline responsibility in Canada, then priority exists (e.g. EVA). Priority is almost always respected in Japan, Switzerland, Singapore, South Korea, Hong Kong and to a slightly lesser extent, Thailand. Had poor experience on 6+ collections in mainland China.
Perhaps Air Canada counts on the sheepishness of Canadians and lack of recourse to offer a service that it does put much effort into.