A useless button which is only intent for mechanic use. In the old days, this was often combined with the Eject button 
We can do with play & pause combination only 
but if you only pause your track your hdd will spin forever.
stop the track and your hdd will stop spinning a few minutes later.
Yes removing the stop button doesn’t sound like a good idea to me, main reason spinning drives (active storage) uses way more power.
But if you would be able to make the storage go to sleep while having a song paused, the button could be removed. Making this work will probably be a pain in the… you know where… You would need to load the complete song into the RAM and play it from there, this would be needed for every song, and adopting this in a way that you can skip to the next song without having to wait…
I don’t know, keeping a simple stop button looks easier to me