It is also good to designate these "orphaned" airline FF accounts as the miles option when renting cars.

As BlondeBomber mentions, the goal is to build up just enough miles in these orphaned accounts so that you can make a transfer to HHonors. As soon as you have enough to transfer to HHonors, you should start working on accumulating FF miles in the next orphaned account. By transferring these orphaned miles to HHonors, at least you will receive some benefit from them.