Pigma Micron makes it effortless to write small, legible characters and consistent underlines. The chemical properties of pigment ink allow it to dry on the surface of the paper as opposed to soaking into the paper. That, combined with the fine nib sizes, minimizes how much your writing, highlighting/underlining is visible on the other side of the page. Those are the two biggest wins when writing on very thin Bible paper. Oh, and the ability to write legibly in very tight margins.
Being able to underline without ink skipping brings a smile to my face, and I am confident it will bring a smile to yours!