I know this question is a bit old but I developed a module to do so  : I have to hide fieldsets and keep the fields value

"#access" have the effect to remove entirely the fieldset from the form, my module only set it to hidden (CSS)

Take a look : http://drupal.org/project/hide_group

I have developed a module to do so: the Hide Group (fieldset) module.

The #access property has the effect to remove entirely the fieldset from the form; my module only set it to hidden via CSS styles.