I think we are having a failure to communicate.... The point of a laptop is portability... you can have your computer wherever you want it...
When you pay top dollar for a laptop (and since this is AMD's fastest mobile CPU, expect to be paying that for anything which includes it), you will be using it for intensive projects - be they CAD, gaming, etc... Those intense projects will usually be performed PLUGGED IN - but whereas you can't move a desktop, you can plug your laptop in at home, work, etc...
When you are performing "light tasks" - like checking email at the cafeteria or browsing YouTube, etc... then you are using your battery - and don't need to use your dGPU...
But if your laptop is ONLY being used for these "light tasks"... you are a fool to be wasting money on the 6900!! The 6500, 12500, (or 3600, 9500, etc) will be more than sufficient.
Once again - if you are springing for the 6900, unless you are getting it cheaper than the 12700, you are wasting your money.