Dont think you would have some use of Max or Maya without their plugins (fume fx, krakatoa, renderman, wich anyway costs a lot, etc)
You already own excellent softwares for everything, zbrush is more complete than mudbox, especially for modeling/brushes/details, mudbox is a little better for the texturing process but not for long i guess
To me Modo is way better than 3ds Max without plugins, its cheaper and more modern.
I guess it depends on what use you have from those software, but as i said your setup is already excellent, doesnt feel like you would need anything else, if not just a render engine.
In wich case, Vray is coming for Softimage before the end of the year probably, Arnold is unknown to me so i cant speak about it.
Mental Ray is dead in my opinion, it has too much to catch up now.
Again it depends what are your needs tho:)