This is a good idea, but the goal of MD's typography system is to provide the ideal spacing out of the box, especially for longform text, though I am open to suggestions for possible improvements as it is a very fragile system.
On the occasion I need more or less spacing between paragraphs I will apply one of MD's helper classes to an element by going to the "Advanced" tab in the post editor and adding one of the CSS classes:
mb-half (single / 2)
mb-mid (single * 1.5)
mb-double (single * 2)
mb-triple (single * 3)
mb-quad (single * 4)
mb = margin bottom which is the "technical" term for "adding spacing" and all values are calculated based on the line height of the Body font, just to give you an idea of where spacing comes from (it's all relative according to your settings).