You definitely would not want air to be going IN through the soundport at the same time air is going OUT through the soundhole. That would result in phase cancellation, which would make your guitar sound quieter.
Paul's Hi-C analogy is incorrect because there are different issues at work in a Hi-C can than in a guitar. Let's suppose you are pouring out a can of Hi-C with only one hole in the can --- no hole for the air to get back in. As the liquid goes out of the can, the can has less liquid in it, and nothing to replace it. That reduces the air pressure inside the can. The more liquid you pour out, the lower the air pressure inside the can. As this continues, the difference in air pressure between the inside of the can and the outside of the can eventually gets so substantial that the air outside pushes itself back into the can, stopping (or at least reducing) the flow of liquid until the pressure differential is reduced to the point the liquid flows freely again. That free flow of liquid then reduces the air pressure in the can, and the cycle continues. That cycle produces the "glug, glug" effect.
With a guitar, the analogy doesn't work because you don't have a continuous flow of anything (liquid or air) moving out of the guitar. Instead, as the top vibrates, the air pressure inside the guitar alternately increases and decreases relative to the air pressure outside the guitar. When the soundboard moves inward, the air pressure inside the guitar increases relative to the air pressure outside the guitar, so the air inside looks for a way out. When the soundboard moves outward, the air pressure inside the guitar decreases relative to the air pressure outside the guitar, and the air outside looks for a way in. In the Hi-C example, you use an extra hole in the can to allow a continuous flow of fluid out of the can and air into the can; in a guitar, we don't want a continuous flow. We want a vibration --- in other words, the "glug glug" is the very vibration that produces sound, so that's what we want.
When air is trying to find a way in, it will come in any way it can --- through the soundport, through the soundhole, through gaps in the binding, or anywhere else it can. Likewise going out. And that's a good thing. Because if air was trying to get into the guitar in some places at the same time it was going out of the guitar in other places, the vibrations would cancel each other out and you would get phase cancellation.
The speaker analogy someone gave above is an interesting one. There are two basic types of speakers that are relevant here --- "bass reflex" and "acoustic suspension." They work on very different principles. A bass reflex is the type of ported speaker people are talking about in this thread. The idea is that when the air pressure increases and decreases in the speaker enclosure, the air in the port acts as an additional speaker. It moves in and out, creating an additional sound source which has the potential to make the speaker more efficient (louder). It is essential that these ports be "tuned," so that the air moving in and out of the port is in phase with the speaker, or else you will get phase cancellation, and that will actually reduce the volume. The same would be true with a soundport on a guitar --- an additional source of air moving in and out of the guitar can make the guitar louder, provided it is in phase with the vibrations produced by the rest of the instrument.
An acoustic suspension speaker, on the other hand, has no port. It is sealed. The principle is that when the speaker cone moves outward, the air pressure in the enclosure reduces relative to the air pressure outside the enclosure, which pulls the speaker cone back in. When the speaker cone moves inward, the pressure inside the enclosure increases relative to the outside pressure, pushing the speaker cone back out. This could be a good thing or a bad thing. If you have designed the enclosure well (which is a bit tricky), the changes in air pressure will help you --- when you want the speaker to move out, the air pressure is pushing it out, and vice versa, making the speaker louder. But if you haven't designed the enclosure well, it will work against you --- when you want the speaker to move out, the air pressure will still be pulling it in, making the speaker quieter.
Which one is better? Frankly, they each have advantages and disadvantages. Assuming both are equally well designed, the bass reflex speaker will have a flat response curve to a lower frequency, but below that frequency, the response will drop off abruptly. An acoustic suspension speaker's response curve will start to roll off at a higher frequency, but it will roll off more gradually. Usually an acoustic suspension speaker will have tighter and more controlled bass, (although it depends on the design); usually bass reflex speakers will be louder in the lower frequencies before they roll off, but will often be a little muddier and boomier in those frequencies.
Guitars do not fit neatly into either camp. They are not sealed, so you never get a true acoustic suspension effect. On the other hand, they are not bass reflex, because you aren't trying to tune to a speaker cone.
Nevertheless, we can learn from speaker design. The fluctuating air pressure inside a guitar is important to your guitar's sound. While the force of the string vibration starts the soundboard moving back and forth, the alternating increases and decreases in air pressure inside the guitar can push and pull on your soundboard in a way that increases sustain. You can also take advantage of that alternating pressure to create more volume through a soundport.
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). But it's not creating vacuum, it's eliminating it. This is the main reason I choose to do ports.
