The Debian patch is necessary to make using tinyxml with STL support easier. The problem with passing macro through compile command is that it is not persistent. So without the patch, any user codes need to define TIXML_USE_STL before including <tinyxml.h> if they wanted to use this version of tinyxml. This patch makes this macro sticky so that user codes don't have to go through this trouble.

Admittedly this is one of the gotchas of tinyxml, and tinyxml-2 solves it by only using "const char*". So yes, the patch should be applied unconditionally.