I could be wrong, but lets say the Alternator is 100A unit. That's 100A x 12v = 1200Watts (1.2 Kw) at full load. A bit of googling suggests alternators are about 50% efficient. So to provide a full blown 100A continous to drive all your accessories, lights, pumps, fans etc would be up to 2.5kw.
Does that sound right? (Auto Sparkies please correct me)
(by the way, I am not arguing with you, if you want to save anything you can, then go for it - but this might be the figure you're talking about saving if what I say above it correct?) Worth weighing up against the possibility of the extra hassle involved that's all.