I noticed that your non-disclosure agreement templates don't include a place for witnesses to sign. Why not?
Generally, what are the advantages of having witnesses?
Most English law contract documents do not require a witness to sign for the document to be valid and binding.
The main general exception to this is documents executed as deeds. Depending upon person and method of execution, a witness signature may be required for a deed to be valid.
A deed, rather than a contract, should be used where there is a want of consideration (i.e. no quid pro quo).
In the case of NDAs, even unilateral NDAs, consideration isn't usually a problem. At the most basic level, the consideration given by one party is the disclosure of information on the promise to disclose information. The consideration given by the other party is his or her confidentiality undertakings.
One advantage/disadvantage of using a deed is that the limitation period for the bringing of a claim for breach is 12 years, rather than the 6 years applicable in the case of a breach of contract.
Leaving aside the distinction between contracts and deeds, another reason to use a witness might be to guard against a claim by the other party that the contract was never agreed - that his or her signature is a forgery. This sort of claim is not, however, very common. If you don't trust the counter-party to an NDA, you probably shouldn't be contracting with them. Nonetheless, there are circumstances where a witness signature may be advantageous - providing you can trust the witness!