Sitting in the middle lane with adequate speed is fine when there is heavy and/or slower trafic in the right lane. The grandparent talks about sitting in the leftmost lane because it's more convient, rather than sitting in the middle lane and using the left lane when passing.
Eh, I'd have to agree that staying in the middle lane and going the speed limit is the smarter thing to do. There are too many badly-designed on-ramps and people that don't understand how merging works for staying in the right lane to be practical, in most cases.
If I do a steady 140 km/h in the middle lane, as mostly everybody else (here in germany), then why am I endangering anybody, if I just stay there? There is still the left lane for the faster ones and the right for the slower. (in the 3 lane scenario)