[Typo3] Coding guideline: When en when NOT to use/build extensions?

[Meinte van't Kruis]

Hi folks,

The following question is bothering me. For certain types of
functionality I was wondering if I should build an full-blown
extension for it or just include a php script with the functionality.
For instance(small example, but it's the big picture I'm realling
wondering about), in building an XML parser, would it be a good thing
to build an extension for it, or just write PHP code that will be
included by extensions that make use of it.

I'm kinda new to all of typo3, but I hope I made my question clear,
any replies are greatly appreciated :).

Thx