Plug-ins are included by core.php through functions in plugin_api.php.
This can cause problems if the plug-in itself requires an api defining global variables. The variable will not have global scope unless defined explicitly.

This can cause hard-to-debug errors.

If there is no other way, I think we should explicitly define global variables, even if they seem to be by default defined outside any scope (example, adding a
global $g_custom_field_type_definition;
in custom_field_api, even if it seems that this variable should be at a global scope).