The clear cap will let you know that there is water there ready to spill out, however can you really see how much water is present.
Is it a small amount like a quart or so. Or is it 10 gallons? If its a large amount what are you going to do about It? catch in a bucket ?.If a bucket will fit beneath discharge pipe...and then what?
A twist on valve allows you to attach a sewer hose without getting your hands wet with sewage water. A cap will likely expose you to the sewage at some point.
