I don't think it is so much the lugs as the rubber compound on the sole. Fwiiw, I wore some Dunlop KT-26 (very light joggers) on our last trip - which was over some pretty rough and rocky terrain.
The sole of the KTs consists of a thin layer of carbon rubber about 2 - 3 mm thick, with an EVA foam base above that. The sole is quite flexible in all directions. The sole was worn pretty smooth before the trip started, and even smoother by the end, but I didn't notice any problems on any of the terrain. The grip of the smooth sole on the very jumbled and angular rock was excellent. I put this down to the use of old-fashioned carbon-rubber.
That said, my wife took some Salomons Extend Lows on that trip and she had no problems with the Contagrip soles. As those shoes had been used for maybe 6 weeks in France and for many trips since, they weren't new either.
> it's not usual to want a trail runner to provide good grip
I really can't imagine that EVER being true.