At the non-towered airport I fly at, pilots when clear of the runway always say "X Traffic, Skyhawk N123 clear of the active." Why not say the runway number? At a one-runway airport, I understand that this could be redundant, but wouldn't it be better to get in the habit of saying the runway number so that--should there be more than one runway--it wouldn't be an issue?
Honestly? It's probably because they heard someone else do it, and thought it sounded cool (or otherwise just got it stuck in their head).
You're 100% right - saying the runway name is, if slightly longer, certainly a bit more informative. It's the kind of thing that can help arriving pilots know which runway is in use, keeping later radio clutter ("Um, uhh, Jeffco Traffic, um, say runway in use?") to a minimum for pilots already on frequency. Even a 'one runway airport' still has two runways (09/27, for example), and it's nice to know which one you should set up for when winds are light.
The preferred terminology is more like "Jeffco Traffic, 3AC, clear 29".