I have an Expression 4 and bought the 'Straight Stitch Needle Plate with Inch Markings' last week. The dealer who is also a machine technician told me not to move the needle when using it. The 4 also has a setting in 'Tools' which you set to tell you you've got that plate on and it won't allow you to move the needle. I haven't used it yet.
So perhaps this applies to the 2 as well. There is also a foot to match that plate called the 'Straight Stitch Foot with Round Needle Hole'.
I bought my plate because it's supposed to stop fabric being drawn down into the dogs:
http://www.pfaff.com/au/14802_3427.html