Having multiple 3D viewpoints of the model is important both for modelling and presentation purposes but currently it is not possible to define any viewpoints in addition to the two generic projections.
Yes - it is possible to save multiple 3D views but the problem then is that the views are without a proper viewpoint source and so multiple views that share the same projection settings but carries different information through different general view settings cannot be efficiently managed with regards to duplication and changes.
Being able to have additional user defined 3D viewpoints directly in the project map would give a better overview, make view management more efficient and also be consistent with how it works for other viewpoints.
Open