Do low spec machines/users even download, play, etc, 3D games? Seems to me those are not your target demographic.
3D games is not a target demographic. There are many demographics within 3D games, and some of them will include low spec machines and some don't. However, broadly speaking, yes, many, many people with low spec machines play 3D games. I wouldn't have gone to so much trouble to make a fixed-function fallback for my current game if people with low spec machines weren't playing my previous games.
EDIT: To be fair to the developer of the Leadwerks engine, it is bloody hard work though, so I can understand why anyone wouldn't want to do it.