I think it would be convenient and user-friendly if there was a way to highlight your text and click on the 'Hidden' or 'Spoiler' button.
It would be very similar to when you're making a signature, you highlight the area of where you want to add a colour or size change and then click the button corresponding.
Again, I support this idea!
However, considering that a lof of people have failed to add own buttons to the text editor successfully (yeah, MyBB sucks in this case), I think that we won't get this feature - at least not for quite some time.