This 'effect' was very handy in
Descent, where you could engage both forward and side thrusters at the same time, making your effective speed equal to the
hypotenuse of the two perpendicular speeds (sqrt(2)=~1.4142) - approx. 41.42% faster than with just the forward thrusters.
Once you had the 'leet chording skills necessary, you could combine this with the vertical thrusters as well, giving you a ~73% speed increase. Throw in the afterburner power-up (which doubled forward speed) and you really could rev that ol' gunship up. You had a very vague idea where you were going, but hey, it looked slick.