Ahh, nevermind then. I thought it was use it all at once or lose it, but checking the T&C shows you are right:

Residual Value: If redeemed for less than its face value, the remaining residual value of this voucher may be applied to subsequent ticket purchases at www.delta.com/redeem. The expiration date of the original voucher will apply to any residual value. Residual value of a voucher may only be used by the original voucher owner.

Awesome.