I might have this wrong, but from what I understand, pre-15.04 versions of Ubuntu Mate (the 14.04 and 14.10 releases) are built outside of the main Canonical structure. That being the case, I think Martin was forced to package everything up in a single bundle. That means that if you try to uninstall one particular aspect of the initially installed system, it wants to take everything else down with it.
I am guessing that the newer version (15.04) will no longer have the above issue. I happen to be using 14.04 myself. The way I have dealt with the issue is to simply leave the existing software in place and just remove its icons from the menu. Then install whatever replacement software I want. I realise this might not be an appropriate solution for you if you are trying to keep your system as skinny as possible. In which case, your solution might be to install the 15.04 version. But, I would just check with Martin first to establish that the problem of the system being all packaged as a interdependent whole has been resolved in 15.04.