Note block is for one purpose only, to make soundsor music. But it is difficult to make because you need to repeatedly punch each block until you have the right pitch. If there's a GUI it will be a lot more better and easy. And also the people making redstone music will have a nice work.
Hmm as Mart_b77 said the GUI would add another thing for resource packs to have to texture. The immersion is much better with fewer GUIs as well. I do like the idea of choosing which note you want specificly though, so maybe an item that you select the note in one way or another and change the noteblock to that note when you right click it with the item. PARTIAL SUPPORT
The two people above raise good points. Here's a possible idea: have a new item called a 'tuner' that can be set a note independently identical to the current note block system, and which can be used on any note block to switch it to that note.