I think flat design is inclusive design, that is, it's not hoity-toity, impossible-to-execute, beautifully 3D-rendered and textured expensive design. It's simple and clean, sparse and understandable. In the most simple application of "flat design" trends, it's essentially one step up from a wireframe: pick a color scheme on some color scheme website, download a 200-pack of glyph icons and BAM you've moved from an abstract wireframe to something a little more sophisticated and approachable. Friendly colors, nice spacing, good typography, and you've got something.
Yes, I am saying it is quicker to execute a "flat" design than it is to execute a skeuomorphic, highly-stylized, deeply-textured design. A product manager or developer without a lot of design experience could probably use some flat design trends and one nice font and put something together that doesn't look like horse shit. I don't think that same product manager could spend an equal amount of time trying to mimic a beautifully-crafted and textured interface and have anywhere near the same luck. Using flat design trends gets you to some sort of product design quicker than if you're spending 20 minutes crafting a 5-color gradient to emulate the perfect lighting of a semi-glossy aluminum rod for your button texture.
No, I am not saying that by simply using "flat" design trends you are able to bypass all processes and thinking surrounding good whitespace, grouping, contrast, alignment, typography, usability, etc., but because "flat" design elements are graphically simpler it lets the content of the design show through and not be muddled by textures or skeuomorphic execution flaws, so your design message is clearer and cleaner by default.
Non-designers or "mediocre designers" seem to be flocking to these flat design trends the same way they flock to the excellent Bootstrap framework: because it's a simple way to jumpstart a product design out of a blank canvas or browser window into something that, even when starting out, looks pretty decent.