Yes, but frequently pilots refuse having anyone on a jumpseat, plus, on really long flight, this is a pain.
And, you would wonder if you knew how many standby people are usually waiting for the flights. I have seen flight with more than 50 standbys and all went home.
There are stories of LH people waiting i.e. in BKK for days and then buying a full fare ticket last moment to avoid being fired by their employer for not being back on time for work.