because less is more! and people on the diy
forums want to squeeze every last watt out of a light we possibly can, and its done with the use of Od'ing. where's the fun and excitement in using 2 seperate fixtures? i know i have to pump out every last Megahert from my cpu and vid card, or i can't sleep at night.
but all joking aside, thats probably the simplest and easiest solution out there, however unless you're dealing with a large tank (55gallons and up) you will be rather limited by the width of the tank, and the amount of bulbs one can use as a result. if 2 bulbs take up 1/2 the width of your tank, 4 bulbs will take up almost every inch and make feeding difficult, as well as some other daily fish routines. also magnetic ballasts do shorten bulb life, generate hums, run hotter, and use (on average) over 60% more power than ellectronic ballasts. and if you are going to use electronic ballasts to skip over the above problems, why not spend 4-5 bucks more and join the od'ing bandwagon ? but if you want the cheapest (and the most crude) lighting system out there, 2 fixtures would be the best idea, if you can work around (or tolerate) the tank width problem.