The major argument against issuing debit cards to access a savings account (only) are (1) the transaction limits applicable to savings accounts under Regulation D §204.2(d)(2), and (2) the fact that some networks don't permit a savings account to be listed as the primary account accessed by such a card.
Even many ATM cards can be used in ATM networks to complete PIN-authorized purchases at participating merchants. Those transactions would be counted toward the monthly 6 per month Regulation D limit, too. However, most banks have found that savings only ATM cards have not been as problematic as savings-only debit cards because the number of places they can be used is smaller.
John S. Burnett
Fighting for Compliance since 1976
Bankers' Threads User #8