Not just because of performance. Gdiplus has some features that gdi32 does not, such as anti-aliasing, semi-transparent brushes, and gradient brushes.
I see.. So basically everything with hdc is old GDI and everything without is GDI+?
If there's an HDC, we use a mix of GDI rendering and GDI+ rendering. If GDI can support a particular drawing operation, we use that. Otherwise, we render it internally and send it to the HDC using GdiAlphaBlend.
At least, that's the general policy. There may be exceptions.