Well first off it's awfully judgmental of you to say only someone with a CFI can handle more than one flight a day, but moving past that, if someone wants to spend their money they will find a place to spend it elsewhere if you won't let them. I mean a student pilot can easily double their flight experience in two days if they do four flights over those two days, also, considering they're not involved in anything else on those days it would definitely get their mind thinking, I've learned a lot of tidbits regarding aviation just by flying along and noticing something and having that become a question in my brain to find th answer to later.

I guess this is what it boils down to... how does it become unsafe, inefficient or illegal for them to fly more than once a day? And "I think" is not an argument.