"it seems that the dependent care benefits provide a tax advantage to married filing jointly if both spouses earn income."
That's the point. If only one spouse works, the other spouse presumably can take care of the child. The credit creates an incentive (although rather small) for the non-working spouse to work if they want to, by reducing (by a small amount) the burden of childcare. (The credit is also available if the non-working spouse is disabled and unable to care for themself, so that childcare is needed so the other spouse can work.)
