While it may be technically possible to port LDD and LDraw-based models to Spring, in practice it's not a good idea - you'd spend so much time optimizing the geometry and re-doing things to lower the polygon count that in the end it's far easier just making the models from scratch. They can be useful for referencing basic dimensions of pieces you don't own in real life, but even then, it can be a hassle, as I've found LDraw bricks to be quite hit-or-miss in the accuracy department.
Would it be helpful to create models in LDD to export to LeoCAD?
To an extent. It varies on a set-by-set basis, and I'm still working out the details.
You're using sets from old themes, right? Do you want them to be updated or modified or in their original form?