In theory I suppose the tether could be equipped with a large-ish rocket that's capable of going out and grabbing anything that misses and putting it back on a

In theory I suppose the tether could be equipped with a large-ish rocket that's capable of going out and grabbing anything that misses and putting it back on a correct course. If you had to use it all the time, it wouldn't be worth constantly refueling it -- you might as well not use the tether at all and just use bigger rockets. But if it only gets used occasionally, it's probably a good insurance policy to have on hand and would just be a cost that gets amortized across all the successful launches.