I have carried a pair of kayaks on my car roofrack for many years with no trouble and always used bow and stern lines to the car towing points. As I understand it the bow and stern lines are primarily to prevent forward and backward shift of the boats, and secondarily to act as a safety in case of multiple strap failures or the roofrack detaching from the car.
My new car has towing points that a) are very inaccessible and b) the front ones are very far forward of the kayak bows. This means that if I take a bow line to them there will be nothing preventing forward movement of the kayaks (the rear tie points are almost underneath the kayak sterns and so are little help.
Therefore I have started taking the bow and stern lines to the car's roof rail, running them backward from the bow and forward from the stern.. It's more convenient, it provides better protection against backward and forward movement, and the roofrail is an integral part of the car so the kayaks will still be attached even if the J-racks fail.
Poor diagram follows. The red lines are the bow/stern lines to the rack mounts, blue lines to the towing points. Not to scale.
Is there any downside to this setup that I haven't thought of?