It's important that painting and splines are different things, just like raster and vector graphics are different things. You typically don't have dozens of splines overlapping one another, but even if you do, their height at any given point is explicitly determined by the spline and is easy to see and understand.
See How do I Edit older paths? for more info on the current workflow.

Why Path Painter is not like a spline tool?
If someone needs a spline based terrain tool, it's best to use one of those. Path Painter wasn't released to be a redundant thing and it never claimed to be another one of those. Quite the opposite.

What can I do if I need spline-like behaviour?
Since Path Painter is not a spline tool, there are currently no built-in "point and shoot" spline features. Support for splines was built into the API, if you want to use splines to drive your paths instead of painting (using a 3rd party spline solution). There are two behaviours available:
  • the typical spline behaviour: where paths' heights are driven by the spline, and
  • paint behaviour: where only the track of the path is driven by the spline, but the height is determined by the terrain and the terrain follow and slope limit settings.
You can find more info on how to use the API for this in the documentation at page 62.