The preview button is useless simply because you already get a preview right away now prior to you manually clicking the preview button. The second you put in the BB code it will automatically make a new preview for you on the bottom of the page where everything is displayed.
If this is unclear, please do let me know and I'll try to explain it better.

This is NOT a suggestion, just a simple removal of an unnecessary button!

Not true, if you are previewing a toxin orb and want to go back to your CURRENT settings on the rainbow you need the Preview button(w/o needing to change the foreground or background color). This was intended to function as it does, no mistakes were made.