It's a good idea to have some objects isolated in our portfolios.
The main reason to isolate is so it can be removed and used elsewhere. So if you use black it easier to do it on a white subject.
Of course any other color than white can still be done using the white background.
The whole point of this is that yes there are rules and guidelines that should and need to be followed.
But it makes sense to stray from them for common sense or just to do it different some times.
It's kind of like saying a subject should never be in the middle of the photo except when it needs to be in the middle.