I would agree that using FG for stamina refill isn't really an efficient way to use them. But that's really all that we can say about it: it isn't efficient.
Efficiency is one way to play (and reasonably the most determing one) but it isn't the only way to play.
Another example would be it's more efficient to always wait for 50 FG since u get an extra pull. I however don't always wait for 50 FG. Since I'm f2p, it takes a long time for me to get 50 FG. But if there is a new gacha which I like, I sometimes get the urge to pull immediately. Yes I could wait till I have enough gems or wait for another good gacha but in the end I realized that it satisfies me more to do a single pull and get a girl I like instead of resisting the urge and wait. Because pulling 2 or 3 new knights somehow doesn't satisfy me more than getting only 1 new girl.

Well I guess this isn't rly the answer you wanted^^ but I just wanted to say that while playing efficient is usually the best way to go for, there are times when you might enjoy the game more if you're not playing in the most efficient way.