This quote perfectly sums up the concept: A modular scale, like a musical scale, is a prearranged set of harmonious proportions.—Robert Bringhurst
Here's a link from early on when foundation was beginning to move in this direction with typography:
http://zurb.com/article/1000/foundation-3-0-typography-and-modular-sca
You don't need to worry out changing line-height or margin-bottom. In foundation, those are all in ems, so basically a percentage of the font size.
I also have moved to doing all padding and all margins in ems, and notice how well all elements on the page arrange and family together as the browser window gets smaller or larger.