" . . .for the B734,autobrake RTO is only armed after 90kt on the t/o roll.just a point.."
Similar for the 'bus - About 70 kts. Lose an engine before these speeds and the asymmetry can take you off the runway very rapidly. A sim instructor used to demonstrate this very well during his sessions. That half second delay in getting your feet in position may make the difference.
History shows that if a manufacturer recommends a particular technique and you don't follow it, you can have a very difficult time in court.