The advantage of using iTunes credits vs credit cards
I am one of those people that would rather fund my iTunes account with an iTunes gift card rather than connecting it to a credit card. As has been mentioned, there will be no surprises on my credit card at the end of the monthly if my iTunes account were to be compromised.
As far as losing interest on the money I have credited to my iTunes account, so what. The lost interest on the $20 average balance in my iTunes account is negligible, less than a few pennies a year.
The one main disadvantage of reloading your account via iTunes Pass ( or linking your account to a credit card) as opposed to buying a gift card is that gift cards go on sale periodically whereas i doubt Apple will ever give you any bonus when applying cash to your account via iTunes Pass.
Best Buy and other retailers sometimes sell $100 gift cards for $85 or $50 gift cards for $45.