There sure is an abundance of these lately. I can see the reason why Bootstrap opens up the possibility of building this, but I don't see exactly where it's going. Maybe I'm missing something though, so I'd be curious to have the creators defend the work.
What is going to make one of these win over the others, and who exactly is the intended audience?
Before releasing the Bootstrap Designer, it was my personal tool to create a mockup for clients. I can quickly experiment with colors, fonts, images, etc and pick the combination that I feel works for my project. I see this will help users to focus more on creative work.