Taking room air to remove the heat from the condenser and exhausting it outside reduces efficiency. When you remove a cubic foot of room air, it has to be made up from somewhere. Then you need to condition that air too.
Using outdoor air to remove the heat from the system allows you to keep that indoor air that you have paid to condition in the indoor space.

I "think" the Whynter normally uses outside air to cool the compressor, pulling it in through the compressor intake duct and exhausting it through the compressor exhaust duct. In other words, it recirculates the inside air through the evaporator, with the evaoporator intake at the upper rear of the unit (where the filter is) and the evaporator exhaust at the top (where the vent is).

If you remove the intake duct, then it uses inside air to cool the compressor and exhausts it through the exhaust duct, which means that inside airr has to be replaced by hot outside via air leakage.

This isn't discussed in the manual, but is what I think based on other reading and having the same unit.

The most effective way to use it is to turn it on in the morning and run it all through the hot part of the day. If you wait until the room is hot to turn on the unit, it will take a while to get the room cool. Better to start with a cool room and keep it that way.