  When CMB data are used to derive cosmological parameters, their very choice
does matter: some parameter values can be biased if the parameter space does
not cover the "true" model. This is a problem, because of the difficulty to
parametrize Dark Energy (DE) physics. We test this risk through numerical
experiments. We create artificial data for dynamical or coupled DE models and
then use MCMC techniques to recover model parameters, by assuming a constant DE
state parameter w and no DM--DE coupling. For the DE potential considered, no
serious bias arises when coupling is absent. On the contrary, \omega_{o,c}, and
thence H_o and \Omega_{o,m}, suffer a serious bias when the "true" cosmology
includes even just a mild DM--DE coupling. Until the dark components keep an
unknown nature, therefore, it can be important to allow for a degree of freedom
accounting for DM--DE coupling, even more than increasing the number of
parameters accounting for the w(a) behavior.
