I have about 25 bobbins for my main machine, and I'll try to use up the odds and ends if possible, and will only pull it off if I need a bobbin and don't have any empties to use. I almost never wind a second color over a first unless I know I won't need much thread to complete what I'm doing (Like mending).
I've gotten many old bobbins with thread on them with machines - and many, many spools of thread. In most cases, the thread is either not the quality I will sew with (some cheap stuff is rather horrid) or it's old and brittle. I've had a lot of thread I've pulled off that will break given a bit of a tug. I know make a point of removing it all off the bobbins. I have enough misc thread spools that I'll never use it up in my lifetime.
And that brings up another way of "wasting" hanging on to something and not using it - and letting it sit outside, or in a shed, or poorly stored so that in the end, something that was once very nice is now horrid.
Or hanging on to it until you die, and then having your kids throw it in the trash because they don't have the time to deal with it.