Field API deletes fields when the last instance is deleted. Field UI populates the "add existing field" dropdown only with fields with at least one instance. This makes it hard for modules to expose fields to a website, and build business-logic functionality tied to that field, without also locking at least one bundle into using (instantiating) that field. I just posted a D7 contrib module, ModuleField, for solving this. Is there any interest in this being added to Drupal core (directly to field.module and field_ui.module)?
PASSED: [[SimpleTest]]: [PHP 5.4 MySQL] 81,173 pass(es). View
FAILED: [[SimpleTest]]: [PHP 5.4 MySQL] 81,163 pass(es), 0 fail(s), and 1 exception(s). View
PASSED: [[SimpleTest]]: [PHP 5.4 MySQL] 81,111 pass(es). View