If you condense everything that can be condensed, the resulting style sheet could be harder to adapt to changing requirements.

One example of this is when some styles are "accidentally" the same for two elements. If you condense them into one rule, then you lose the independent control.

"Hey look, 97% of the users always turn up bass and treble together and turn down the middrange; we can condense the three-band equalizer panel down to one knob!"