Hey Greg! how are you doing?
Yes, that's actually something I was thinking as well. All those things are implementable in compositing packages if the proper buffers are exported - even spot replacement effects can be implemented with a wp pass, but the advantage of solving some of those things at pixel level at render time should be noted too.
For example, a very shallow Depth of Field filter applied at pixel level in render time offers better results than applying it at image level within a compositing package. Reason for this is we are able to filter some few 3D DOF passes and get data from what's behind the foreground geometry without rendering 2 layers (FG/BG) by separate, so results are not only better but also we can use less AA passes than solving it with 3D DOF solely. You'll probably remember X-Dof worked in that way. In similar way, it's possible yet with 2 instances of DP NPF. Being able to do it in node editor allows to configure some things in a flexible way without any programming skills. There's also other scenarios where we might want to render at a specific reference white and perform post white balancing according to another illuminant. Settings things up at pixel filter level may be more flexible for changing easily the reference white, instead of changing HDR lightprobe and each lights separately. Some other effects would be better applied after surface evaluation like a kind of global shader node editor, I guess.
Good thing is that having these things working at both pixel and image filter level allows the user to have some boosted degree of post-processing power within LightWave. So for quick concept designs, look development, storyboardings, animatics, art previz or just for selling quick preliminary ideas to a client, solving all these render things within a single package is really very handy.
Cheers!
Gerardo
p.s. Just noticed in case you were referring about the specific case of the method shared here, only the second filter is implementable in compositing package, easier with a macro or gizmo.