The only real way to measure any advantage is against it's respective disadvantage IMO. Comparing 20/4 to 18/6, using one 400W bulb 365 days a year, would cost (by our prices now) about between $20-30 a year depending on when those 2 extra hours were used. I'd say that 2 hours extra electricity is a negligible cost for the average home grower. Now, running 10 - 1000W bulbs, 2 hours less per day would save about $50 / month (keeping in mind that is running 365 days a year).
Out of experience, have you guys seen any noticeable difference in yield between running 20/4 and 18/6? Would an extra 2 hours of light / day yield more than $600 in extra weight per year under a 10-1000W set up? If yes, then the extra energy costs would just be an investment, rather than a disadvantage. If not, then it's a clear disadvantage.
I can certainly agree that ballast and bulb life would be two definate advantages! Which is why I'm not running 24/0...