That certainly hasn’t stopped people from trying to build their own homebrew polyline classes to plug into MapKit. In particular, Craig Spitzkoff wrote up a document describing a method of displaying a polyline on the map using an annotation, albeit not updated in real time (the polyline would disappear when the user is performing a zoom action on the map). Nicolas Neubauer later expanded upon Craig’s idea with the idea of hooking into the annotation view’s center property (called by the map kit view) to get notification to resize the polyline appropriately on zoom and move events. Unfortunately his description has a few errors (you need to hook into MKAnnotationView, not MKAnnotation), but the general idea was correct and was a rather elegant hack around the limitations of MapKit.
Go ahead and checkout the repository on github. You know you want to.