This Button solves most of the problems but not all of them. Especially when using different Fonts on a Website for Bold and normal Text both fonts exist after using this button too. It looks a bit strange then and users have no tool to correct this. Having a Font Dialog will give the user a tool to "repair" such things and will give them the ability to change fonts where they need it.
It is understandable that you disabled font for stability-reason and it surely is no bug to have just one font, but it would be nice to have that opportunity to set fonts back in the mail-Dialog. Perhaps the users could help to identify the instability and to remove the reason!?
an Apple a day keeps Dr. Watson away