When sound energy is directed upwards to the player, then that amount is missing from the forward direction (so projection to the audience will be lessened).
Therefore, I don't think it'll become a standard feature because it is going to be viewed as a compromise.
Also, the sound from the side portal may give the player a much different tonal characteristic than the audience because of the location (and some might view this as a compromise).
If the side portals include plugs, then most of the compromising issues are resolved, except for the extra parts, extra weight, and possible changes to the structural integrity/tone (although a properly engineered structure with holes can be stronger than one without, so maybe a benefit is a stronger frame).
An interesting benefit of the side holes, if big enough, is that they can provide extra gripping spots for the ukulele during transport, assuming a strong overall frame.