This setup would probably be because of class rules. Ive seen it a bit(DCOE`s only using half). It is often because the car doesnt require the extra carburation but the Weber is easy to tune and get parts for.

It is interesting when comparing ITB`s (IR) to single point delivery. An engine with twin 40 dcoe`s and 34mm chokes would perform reasonably well swapping to one 34mm choke on a common plenum. This is because each cylinder takes its turn at using the carb. its just used four times as often, albeit, with a greater retriction right at the top end.
The advantages with Inividual runners is the freedom to alter their lengths easily, having less bends and to prevent charge robbing on cars with high overlap cams.

A car will often perform better( higher average torque) with a carb/s that is/are a bit too small than way too big.