Given the fact that an exact straight line can be generated with six or more links using
only revolute joints, why use a fourbar approximate straight-line linkage at all? One
reason is the desire for simplicity in machine design. The pin-jointed fourbar is the simplest
possible one-DOF mechanism. Another reason is that a very good approximation
to a true straight line can be obtained with just four links, and this is often "good enough"
for the needs of the machine being designed. Manufacturing tolerances will, after all,
cause any mechanism's performance to be less than ideal. As the number of links and
joints increases, the probability that an exact-straight-line mechanism will deliver its theoretical
performance in practice is obviously reduced