portal.io just added this feature and I think this a huge game changer. You give the customer the ability to modify the proposal and add up-sells /cross-sells on the fly. Give the option for good/better/best A/V Receiver for instance and they will generally go for the Better option. If you have to send them 3 different proposals to give them those options they are much more likely to go with the least expensive fixed proposal, then if you gave them option to make the selections on the fly.
this would be great so they can just uncheck items or check items, along with A/B option. example hide wires / wire mold.