Correct me if I'm wrong, but it's also the addition of a boatload of adjunct grains - mostly corn and rice -that effectively removes most of the *beer flavor* from BMC and BMC Lite, right? You'd want to find some adjuncts that are highly fermentable, so that you don't have a lot of residual sugars around to make the mouthfeel heavier. Fewer unfermented sugars means fewer calories, as well. I'm certainly not the expert in this matter, but I do believe that it's more complex than just watering down a regular beer.
Of course, the end result will probably taste pretty close to the same.
Most light beers are lagers, are they not? Makes the fermentation process a lot more complex, you'll have to be able to keep temps low for an extended period of time for the yeast to work properly. Something else to keep in mind.
Nothing wrong with a commerical beer on some occassions (for me, not Coors Light, but I'm not one to judge). My guilty pleasure is Molson; nothing better with a plate of Buffalo wings than a bottle of Molson Canadian. I agree with Fatabot, though, that if I'm brewing, I want to make something a little more complex, something that I'm apt to savor. "Drinkability" isn't a key consideration, I'm not drinking homebrew to get drunk (I *might* be the exception around here). Buy the cheap beer for when it's hot and when you want to drink a lot, brew the *good stuff* for when you want something a little more complex.