I don't think I've been bitten by an add-on item since they've introduced it. A food processor would definitely get you free shipping in two days via Prime, it is small items like a 6oz bottle of lotion that costs $1.99 that you can't get by itself.
And to your point about attaching them to a prime order, I would have to imagine Amazon is aware of this limitation and I wouldn't be surprised if they add this feature soon.