In reply to Dougal Campbell.

The problem seems to be that most people do not know how WordPress stores the tags in the database. They look for a table like wp_tags and functions like get_all_tag_ids(). They do not connect "tag" and "taxonomy" and therefore a function called get_all_term_ids would not help them.

Nevertheless such a function would make the logic of the WordPress functions more consistent. Sounds like a good idea.