www.kraytracing.com

I mean : why using picturenaut or qtpfsgui instead of internal Kray solutions for tonemapping would be a bad choice?

Personnally I prefer tweaking a render in postprocess, but I have no idea about realism accuracy and quality of such a pipeline, comparing to the one offered by Kray.

there is nothing against that, I also render to HDR and tweak in f.e. AE sometimes.
But the present QLWF plugin with coupled postprocess settings in kray itself tonemapping to 24bit, makes it a lot easier to get good results.

You can Tonemap afterward, of course, but you have to know that depending on what you choose, you might need to change your textures before while Kray QLWF does it for you.