  Gravitational waves detected by advanced ground-based detectors have allowed
studying the universe in a way which is fully complementary to electromagnetic
observations. As more sources are detected, it will be possible to measure
properties of the local population of black holes and neutron stars, including
their mass and spin distributions. Once at design sensitivity, existing
instruments will be able to detect heavy binary black holes at redshifts of
$\sim 1$. Significant upgrades in the current facilities could increase the
sensitivity by another factor of few, further extending reach and
signal-to-noise ratio. More is required to access the most remote corners of
the universe. Third-generation gravitational-wave detectors have been proposed,
which could observe most of the binary black holes merging anywhere in the
universe. In this paper we check if and to which extent it makes sense to keep
previous-generation detectors up and running once a significantly more
sensitive detector is online. First, we focus on a population of binary black
holes with redshifts distributed uniformly in comoving volume. We show that
measurement of extrinsic parameters, such as sky position, inclination and
luminosity distance can significantly benefit from the presence of a less
sensitive detector. Conversely, intrinsic parameters such as
\emph{detector-frame} masses and spins are largely unaffected. Measurement of
the \emph{source-frame masses} is instead improved, owing to the improvement of
the distance measurement. Then, we focus on nearby events. We simulated sources
similar to GW150914 and GW151226 and check how well their parameters can be
measured by various networks. Here too we find that the main difference is a
better estimation of the sky position, although even a single triangular-shaped
third-generation detector can estimate their sky position to 1~deg$^2$ or
better.
