I added a shoulder strap to the folding basket recently, something I'd had in mind to do since I got it. I tend to put a messenger-type bag inside the basket and if I need to go shopping I can put the messenger bag over my shoulder and the folding basket is empty. Having a shoulder strap for the basket (or whatever luggage) is crucial for certain things, i.e. if I need to completely fold the bike and carry it having the basket on my shoulder gives me a free hand for opening doors etc..
I've got the Carradice front bag but I never use it as the basket is generally more useful.