the limitation to webserver outputs is for security reasons. With a full access anyone spying your pw would have full access to machines controlled by the RevPi.
It is up to the customer to write own web based sw with any kind of system interaction and then use these webpages for remote access. You could think of a web based UI to even get very deep control over system functions but limit any violator to the functions you have programmed instead of opening a barn door to the full OS access.
If you are not interested in such a security I would recommend you rather go for realVNC which is part of the Jessie image. You just need to activate its function as described here:
https://www.raspberrypi.org/documentati ... ccess/vnc/
This works fine even over the internet but you need X running (i.e. you need a GUI on you RevPi) in which you could e.g. start a terminal window to have full root command line access to the system.
But if your pw of the realAccess account ever gets into wrong hands your machine is fully naked!!!
So please be careful if this is a use case with critical things being controlled by the RevPi.