  Although it is accepted that perfect-merging is not a realistic outcome of
collisions, some researchers state that perfect-merging simulations can still
be considered as quantitatively reliable representations of the final stage of
terrestrial planet formation. Citing the work of Kokubo & Genda [ApJL, 714L,
21], they argue that the differences between the final planets in simulations
with perfect-merging and those where collisions are resolved accurately are
small, and it is, therefore, justified to use perfect-merging results as an
acceptable approximation to realistic simulations. In this paper, we show that
this argument does not stand. We demonstrate that when the mass lost during
collisions is taken into account, the final masses of the planets will be so
different from those obtained from perfect-merging that the latter cannot be
used as a valid approximation. We carried out a large number of SPH simulations
of embryo-embryo collisions and determined the amount of the mass and water
lost in each impact. We applied the results to collisions in a typical
perfect-merging simulation and showed that even when the mass-loss in each
collision is as small as 10%, perfect-merging can, on average, overestimate the
masses of the final planets by $\sim 35\%$ and their water-contents by more
than 18%. Our analysis demonstrates that, while perfect-merging simulations are
still a powerful tool in proving concepts, they cannot be used to make
predictions, draw quantitative conclusions (especially about the past history
of a planetary system) and serve as a valid approximation to, or in lieu of the
simulations in which collisions are resolved accurately.
