Dell already used proprietary graphics format before in their Precision workstations (e.g. M6400, M6500) before they switched back to MXM starting with M6600. Since Nvidia stopped making new MXM designs and the last one is power limited to somewhat over 100W, Dell will introduce the DGFF design into it's Precision line as well for the Quadro cards used there.
One benefit of the dedicated card, besides upgrades, is that if the card dies (which isn't such a rare occurrence), it can be relatively easy to replace it. But, when you have your new thin and light laptop with a soldered GPU and CPU, you'll have to buy a new motherboard with both CPU and GPU on it and this costs like a new laptop almost.
Plus, if you have a G-Sync screen, it's connected directly to the dedicated card, rather than the integrated Intel one, so you can't use your laptop with a dead GPU.