I've never seen Empire troops armed with bows, but there were 'crew packs' for a few races. Generally they were just 2 sets of the complete crews from one of the races' war machines (eg. 2x crew from an Orc Rock Lobber, 2x crew from a Bretonnian trebuchet, and so on).
I've never known them to be unique sculpts- any chance of a photo? They may be mis-labelled or otherwise interesting.
As for rarity, they're usually nothing special. A collector may want some if they're NIB, but otherwise they're just a copy of a pretty common model (obviously some, like the Bretonnian crew, would be more sort after than the standard Empire crew).