To understand this, it is quite helpful to know how a differential in a 1:1 car works and why it is necessary. Here is a decent explanation including a movie of an open diff:
A car requires the outside tire to rotate faster while turning to keep up with the inside tire. If this does not happen, then you will drag the outside tire around. However, because of this open diff, if one tire slips, then all of your power is being sent to that tire and not to the one with traction (because the slipping tire will spin easier).
A spool or locker will force the same amount of power to each tire. It negates the differentiation of the power that you see with an open diff. It is bad for turning, but good for forward motion. This is why I choose to lock the rear of my RCs and 1:1s and leave the fronts open.