I find this thread a little hard to follow. The heading appears to question whether payment of NIC is necessary (and desirable if optional) under the circumstances, but most of the subsequent correspondence is a discussion of potential income tax liabilities, deduction of income tax under PAYE, comarison with Income tax personal allowances and recovery of overpaid income tax, but not addressing the NIC question originally raised.
As I read the OP, the husband appears to be paying NIC as a self employed person, which I expect is through the class 2 route rather than purely voluntary class 3 contributions.
Class 2 contributions are normally compulsory if you are of working age, based in UK and engaged in self employment. There is no exemption available simply because you have paid up sufficient years for a full pension.
However where, as in this case, self employment profits are sufficiently low, you can claim (on form CF10) exception from payment of class 2 NIC if you wish. To that extent, if the circumstances fit, class 2 is effectively "voluntary", although once paid I don't think it can be reclaimed if the original payment was valid. Claim form here
http://www.hmrc.gov.uk/forms/cf10.pdf
The earnings limit for exception (net profit after expenses) is £5595 in 2012-13, rising to £5725 in 2013-14, as per here:
http://www.hmrc.gov.uk/rates/nic.htm
Hope that helps