The second benefit of the furring strips is that it leaves enough space to run electrical without drilling through the joists. As we found out running wire upstairs, the 100-year-old framing in this house is practically petrified and very difficult to drill through. But with the furring strips, we can run the electrical along the bottom of the joists and save a lot of time, muscle, and drill bits in the process. We thought the additional cost of the lumber and slight decrease in ceiling height was a worthwhile tradeoff given those benefits.
NB: The formaldehyde-free R-30 insulation that Lowe's now carries is much improved from the last time I bought it. This stuff is a mottled brown and white color (the older version was bleached white) and looks dirty, but it does not explode in a cloud of fiberglass shards when you touch it. I wore long sleeves and gloves during installation, but no dust mask or goggles. With the previous formaldehyde-free formulation, my skin, throat, and eyes were itchy for a week afterward, even with a dusk mask, goggles, and long sleeves. With the new insulation, I felt none of those side effects. So thumbs up for Johns-Manville for improving their product!