Mechta
And therein lies the problem: Addon's often carry with them addon's of their own, that may or not be useful/malicious/just plain intrusive - Its part of the Monetisation of the software developers.
I've used XYX addin for Windows Blah,Bhah since 2002 and now that I've upgraded, I'm totally lost with the new interface, etc etc......
An Addin is a potential vector for Malware, sadly. If you do not like the interface and some systems provide for a multiplicity of interfaces; then it might be better to change the operating system if possible.
Metro on Windows and "Unity" on Ubuntu 13.xx and greater will be, IMHO; consigned to the dustbin of history of computer interfaces.
Functionality of purpose, must be the prime driver in the development of a user interface. Metro and to an extent Unity, were were regressive steps. Anyone remember "Clippy" or that Brown Dog in Windows search.
CAT III