If you give guests total control, they will only use the solution when it most benefits them.
If you're obligating a guest to buy the
DDP for their entire length of stay, they won't mind so much using a TS meal credit for an inexpensive breakfast buffet.
But give them a la carte dining credits and every single dining situation is scrutinized for it's value. If a TS meal credit was priced at a flat $28, sure you'll buy credits for use at Chef Mickey's or Crystal Palace dinner which normally runs around $35. But nobody...NOBODY...is using a $28 dining credit to pay for the $16 breakfast buffet at Trail's End.
Price snack credits at $3 each and people will use them explicitly for $4 popcorn boxes and cinnamon rolls, not $1.20 bananas or $2.50 sodas.
With any a la carte system, Disney loses the week-long commitment, there is very little chance of credits being used for purchases less than their face value or of credits ever expiring unused.