Not at all a waste. There is a weekly challenge for giving a number of commendations. So being able to give to them is a good thing for the giver as well.
i think the point they are getting across to is that the people leaving them don't want the comms, not that they are pointless or don't mean anything to anyone in the game, like for example i main a tank, i get a lot of comms i already reached achievement milestones for comms and im only a few hundred away from reaching 5,000 comms as of writing this, now that i got the rewards i don't wait at the end of runs for a com, ill just leave as soon as i can because id rather the comms go to someone else like the DPS who get less of them majority of the time, if you add a system to comm people after they leave then youll be comming someone who no longer needs them opposed to giving them to someone else whos still in there hanging out hoping they may get 1 anything over 3,000 doesn't have any benefits, sure you could tell people not to comm you after, but chances are they already chose before you can let them know, therefore leaving before you can be voted for just ensures that someone else will get it and takes less time than writing out to not give me it, that doesn't mean it couldn't be done though without being a waste, they can just change the system to allow for people to be voted even after leaving but also have a player setting somewhere which you can set to allow people to commend you after you leave or if you just don't want any at all to exempt yourself from them completely, that would probably be a better system.
It doesn't matter if the recipient doesn't care about comms. The givers care.
I don't really care about getting commendations either, but I also don't think people should be prevented from giving me one just because I didn't want to wait around for them to watch cutscenes, or whatever.
It matters because adding the ability to comm people who have left the instance already necessitates additional data integrity precautions. That is, whenever you enter or leave an instance, your character data is transfered from the world server to the instance server or vice versa, and comms after the instance server transfers your character data back to the world server have to be dealt with a certain way to make sure that they get counted correctly. In allowing this it's possible to introduce a situation where someone tries to give a comm during the transfer, which can lead to the comm not counting or possibly counting twice, depending on the specific timing of how things get handled.
It's simply not worth the time spent coding and testing this thoroughly when the person who would be receiving the comm literally doesn't care. The people who want to give comms can find other people to give them to.