When you add an item like a Notes page, you do so with a text field that is unlimited. The field allows the user of your form to simply begin to type and type and type. They are able to transcribe obits, tell you family tales, and add details that flesh out the boring but valuable facts they've provided for you in other form fields. What the user of your form can't do is spell check.
An unfortunate by product of enabling protection is that it disables the spell checker along with several other items. The formal way around this is to add a macro. I'm going to give you the address of an MVP site that provides instructions for adding a macro (a set of instructions that Word completes automatically) that enables spell checking in a protected form: http://www.word.mvps.org/FAQs/MacrosVBA/SpellcheckProtectDoc.htm.
MVP (Most Valued Professional Microsoft) is a group of Word users who are worse than I am about digging up details. When I'm really stuck, I check MVP to see if they have a suggestion to solve my problem. They frequently do. However, in this case, I think that the solution is worse than the problem.
Here's what I do. In the email to which I attach the form I include instructions. I explain that the creation of the form disables the spell checker and I ask the person filling the form to write their notes in a separate document, spell check it there, and then copy the notes and paste them in the Notes page form field.
This method isn't as elegant or seamless as enabling the spell checker; however, it has a few advantages. First, I don't have to deal with a macro (VBA programming!). They can be quirky and do odd things no matter how careful you are when you construct them. Second, they can cause messages to appear that will confuse and frighten many an end-user of your form. Third, macros are a quick way to spread viruses. When I get a document from someone I don't know and Word asks me if I want to enable macros, I say, "Hell no!"
So there you have my opinion and the workaround I use. We'll continue to look at forms and the many things you might want to consider using them for or not and why not.