The disclosure form is the way the realtors protect themselves. The purpose is to be able to prove that they actually did disclose what the law says they should; it may also serve as a proof that they met you and possibly introduced you to the property. (The later would not usually be related to an open-house situation, assuming it is a listing agent who shows the property.)
Now, would it hurt you to sign? Probably not. The realtors would certainly be happier if you do sign. And, as pointed out in the thread, it would be one less headache for them. So, while you absolutely are not required to sing the disclosure form, it probably will not hurt you to do so... and help a fellow human being who is presumably working hard to earn their daily bread.
Many realtors are talking to their potential buyers in such a way that the buyers are often lulled into believing that the realtor works for them. This why the law exists: the realtors must disclose that they are actually working for the seller, and despite often saying that they help buyers, they are legally obligated to look after the seller's interest if seller is paying their commission.
Do you need to show the ID when entering an open house? If I'm the owner, I'd ask that of my listing agent, and every other realtor bringing people in. The truth is, in today's "buyer's" market, the realtors are happy to have people looking and seldom ask for the ID. Kudos to those who do.