  Recently, the WMAP group has published their five-year data and considered
the constraints on the time evolving equation of state of dark energy for the
first time from the WMAP distance information. In this paper, we study the
effectiveness of the usage of these distance information and find that these
compressed CMB information can give similar constraints on dark energy
parameters compared with the full CMB power spectrum if dark energy
perturbations are included, however, once incorrectly neglecting the dark
energy perturbations, the difference of the results are sizable.
