Short answer: no, homodirectionality (of drop and strike) is not a
requirement for power development.
Remember that while the classic external drop (as in the close combat
or Jack Dempsey drop step) is directional in that it relies partly on
translational transference of mass over distance (a "falling step"),
the GC drop can be done in one place, straight down, with the power
being channeled in any direction (think Fright Reaction).
In many cases, a drop into a "far" leg (ie moving your mass away from
a target) can be converted into a straight down drop into the far leg
(or both legs), rather that a mere falling step into it. This creates
power potential equal to any other drop. The problem then becomes one
of positioning and penetration. Are you still close enough to your
target to hit and penetrate him optimally? Is he moving in such a way
as to make the strike more effective?
See the Combat Boxing DVD, the drill that Tommy Barnett teaches.
That's a perfect example of dropping into the rear leg to send power
forward--into an advancing enemy. Also examine the basic version of
the CCUE, where you drop into the outside leg while chopping to the
inside.
Hope this helps! Good luck!