Hi,
You really only need to have hidden fields on the form if you want to dynamically show them at some point, or if you have some JavaScript that needs the value.
If you want to pass values to a plugin that are not being updated on the form, you simply need to define a pre or post image and register it against the plugin step. msdn.microsoft.com/.../gg309673.aspx
Does answer your question? CRM Forms are unlike normal HTML forms in that you shouldn't interact with the HTML input fields themselves, but rather use the SDK Xrm api.
hth