Ryan, IMHO most folks who say a ski is "too soft" are playing the Macho Card. Saying the ski is "too soft" has become a bad shortand for "I'm such a burly and powerful skier that I need the stiffest boards available."
A truly good skier could ski on a noodly beginner ski and not have any problems. The skier would adjust his/her technique to fit the ski's requirements.
Every ski has its own set of skier input requirements to perform properly. A good skier knows how to "listen to the ski," interpret what the ski "says," and then give the ski what it asks for.
A bad skier blames the equipment instead of looking in the mirror.
Now, with that said...
Softer skis are less efficient at transmitting your movements. If your movements are refined, a stiffer ski gives you better performance because it translates your pressure/input directly to the edge, and translates senations at the edge more directly to your feet.
A softer ski deadens your input and doesn't give the same quality feedback from the snow.
To further complicate the matter, a ski may be soft in one of two directions - longitudinally (tip-to-tail) and torsionally (resistence to twisting around an axis drawn from tip-to-tail along the ski's center in cross-section, sort of like wringing a towel).
A ski also may be soft in relative degrees dependent upon the position along the ski's length. For example, Salomons recently have been made with softer shovels and beefier tails. Your X-Sceam 9 and the X-Scream Series are good examples of this design. Both skis have good torsional stiffness, despite their softness in the shovel.
I have found that among the majority of skiers who talk about a ski's "stiffness" or "softness," most do not know what they are talking about, and this fact is revealed when you ask them more detailed questions about exactly how they believe the "stiffness" or "softness" exists and is demonstrated.
Did that cause enough confusion to make you drop the issue?