We want your logo to be at the top of the hierarchy. It's a part of your brand that needs space to breathe and be easily identified. Positioning the logo next to a heading wih the same font pulls the attention away from the logo and unbalances the design. We choose supporting fonts that give the brand personality. Giving us multiple tools to take your brand even more powerful.
Rules are made to be twisted though, we never tie the hands of our designers. There are occasions, for legibility and accessibility, that we do use one font. For customers with disabilities, it can be detrimental to use multiple fonts.