Showing/Hiding vs.Enable/Disabling form fields

I would like to know the benefits and drawbacks regarding to form design of "dependent fields". For example, in my form I have a Marital Status field, and depending on it's value, I need to know the partner document number and partner name.

• What would be the benefits/drawbacks of not showing the partner fields, and show it on the fly, depending on the selection of the Marital status?

vs.

• What would be the benefits/drawbacks to let the fields disabled on the form, and enable it on other fields selection?

Also others solutions for the problem would be appreciated.

The form is a Web form for a business Web Application, not a web site.