Sorry, this sounds to me like someone is bitter about being invited. "Forced to provide wedding gifts"? Really? I've never felt "forced".
And wouldn't giving the guest the opportunity to choose their own outfit mean they can pick something out that isn't a "one use wardrobe"?
And you have no idea what their reasoning is, so you have no clue whether its unreasonable or not. All of the thoughts here (for photos or to distinguish the bridal party) are nothing more than guesses.
If you don't want to go, then don't. As has been said throughout the thread, its an invitation, not a summons.