If you're going to use a grid, it's typically a starting point, when you're looking at a blank canvas, blank screen, blank piece of paper. And as your work takes shape, you no longer need the grid. The finished work may still have some pleasing hints of underlying structure, even though the grid has been taken away. You may want to shift things just above or just below alignment to create a little interesting tension, which you can see in Cezanne's work. But usually the grid is completely gone by the time you're done. Most of the time, it's not there to "rule" the composition from start to finish. The grid is typically a reference point.

Anyway, my real complaint about grids is all the technical debt it adds, and in the case of Bootstrap it's bad, in my opinion. And it's totally unnecessary. If you want two columns, just work from 50%, you don't need a complex system for that. It's harder to make changes when there's a cryptic grid system in place. When I see code like class="grid-12c" next to class="grid-9b" I'm thinking what the hell? Is that supposed to be human-readable? To me, it seems like a bad trend. If people want grids so bad, then why did we stop using tables 15 years ago? Seriously. I thought floated divs were a step in the right direction, more akin to how designers/artists work, like I can just imagine Matisse saying, "Float that margin a little lower. Perfect!"