My anecdata seems to confirm this as well. We ordered ~30 Macbook Airs at work over the last 18 months or so (startup, so no actual IT department), and I didn't even know they only supported one monitor until a month ago when a new employee asked why their second screen didn't work.
This was the 27th MBA we deployed, so 26 people only used one external monitor (and I went back and asked if anyone needed an upgrade for dual-monitor use in case they just ignored it). All non engineering roles though.
