You're not factoring in increased MP expenditures during dissipation with your analysis. If you're dedicating your AFs to energy drain, then you're forced to heal with spell's that cost MP. Not only that, but you also have to make up the potential healing from the missing fairy, both from embrace and tether. Also, if you don't or can't wait to spend AF before fairy is back, that's future MP free healing you're loosing out on.