Currently there isn't really any incentive to run a regular full node, unless you need one for a business reason.
Bitcoin is suffering from dropping of node participation, but that will not be a problem in Dash because all masternodes are full nodes, so there are always going to be enough full nodes. Additionally, going forward as the network scales and Evolution is released, there really won't be much of a need to have regular, non-MN full nodes anyway. If the blockchain grows to hundreds of gigabytes, users and businesses shouldn't be expected to keep up with that like they have to in Bitcoin. Instead they will be able to access the blockchain served via the DAPI on the masternode network