  We show the importance of properly including the perturbations of the dark
energy component in the dynamical dark energy models based on a scalar field
and modified gravity theories in order to meet with present and future
observational precisions. Based on a simple scaling scalar field dark energy
model, we show that observationally distinguishable substantial differences
appear by ignoring the dark energy perturbation. By ignoring it the perturbed
system of equations becomes inconsistent and deviations in (gauge-invariant)
power spectra depend on the gauge choice.
