You could have doctorate in one discipline, but if you want to make some cross schematics, you will need manuals for the discipline you are lacking.
You might still need a manual. I have learned all 7 schems in Electricity but as my IN is 17 (I used mind marvel to learn 7th) I don't have 100 in Electricity. IIRC it's 90 in my case. So if I want to make Tesla rod I must either have tech manuals or use mind marvel again. And as it was already sadi you need them for disciplines in which you didn't specialize and which are required for custom schems.
And so Intelligence goes with discipline. I never knew that. Well, I knew it has got to do with discipline but I dont know was you need it for the degree thing.