The RM2's wording is, honestly, very open to interpretation, as it merely says that such a shield "functions as a normal shield". A GM may consider it means it works in all aspects as a normal shield, except that it was created by a spell and is invisible, so, once created, it encumbers as a shield would, including the need to be held.
As far as I'm concerned, it's how I play as a shield that doesn't need to be held, thus may be used with a polearm or a two-handed weapon, is a bit too powerful for a level 2 spell that is omnipresent in spell lists IMO.