Beside the security reasons one should also consider the performace issues that may come up by allowing every snip-author to write his own script macros.
The macros are executed everytime a page is rendered, so on a middle range server you could get quickly in trouble.
IMHO there are only few reason's to have scripting abilities. OK, the first of them is: 'It's cool and we have it!'
The plugin architecture is more reliable and easy enough.