Is the weir necessary in the skimmer?

The weir door is not a necessity - however it does help prevent debris that's accumulated inside the skimmer basket from floating back into the pool when the pump shuts off.

I guess if you are really really good about keeping baskets clean ORRRR if you run your pump 24/7 - then I guess the weir door is useless. But rarely does anyone do either! So I would keep the weir door in!

Improvement suggestion 6-11 The weir increases the water flow into the skimmer by reducing the opening of the skimmer. The increased water flow occurs at the surface and this improves skimming of oils and debris from the surface of the pool. A weir may or may not be required by the code officials. It is not a disaster if it is missing but it is a desired feature.