They seem useful for prototyping design decisions, but less so for structural needs. Ie, i'm pretty bad at design, i can iterate to something i like, but it's slow and not something i enjoy. If i need a design for a side project, i'm not going to want to spend much (any, really) time thinking of fonts, colors that work together, etc. Frameworks can make that easy, atleast.

Also, there are elements of design in structure, too - ie how consistent you are with spaces between elements. I don't really want to think about any of these.