That is a valid reason to going for 4 ohm cabs, but not everyone has a power amp or 2 separate heads.
Question for you. If using the LMII with 1 cab runs at 500w, does running 2 with your parallel box have any volume increase? It's a genuine Q as I've never tried it & I'm assuming (from doing some of the maths) that you'll have @ 175w going to each cab. I work out that if a 4ohm cab gives a 3db increase over the 8, & 2 8ohm cabs in series gives a 6db increase due to more drivers moving more air, then in theory (without thinking too hard) you probably dont have a big increase if any?