I would "waste time" on them; it is not only my obligation as their owner and caretaker, it is also financially sound to keep them around until weaning if the cow can be persuaded to take the calf. It would of course put the cow on the cull list, usually it is an economical advantage to cull after weaning time.
Now, if putting them together does not work out it may be advantageous to sell the calf to someone prepared to bottle feed or who has a nurse cow, or it may be possible to graft the calf onto a cow who lost hers. In such case there may be reasons to sell the cow earlier.
In any case cows like this do not deserve a second chance next year.