In some cases, yes. In most cases, no. The level of incapacity doesn’t have to be as severe as a coma, or persistent vegetative state. I have regular contact with people who have a condition called disphagia. It means that their oral neuromuscular control has been compromised. With some, soft, or even pureed foods are the only foods that can be eaten, and liquids have to be thickened to a honey consistency. If that regimen is not followed, they are very likely to aspirate their food or drink, and can become infected with pneumonia. It is a very common cause of death in the elderly, and the infirm.
In the extreme case, tube feeding is the only safe way for them to ingest food and fluids. The tissues of the mouth are treated with swabs, often flavored with liquid food flavors, but never so saturated to present a swallowing hazard. The person may be conscious, and entirely rational. The problem is not always related to the same neural regions that control the conscious mind. In some cases it is entirely related to trauma or infection of the oral motor nerves.
Comatose patients cannot ever be given liquids by mouth, for the same reason. They are simply not able to reliably swallow without high risk of aspiration, although a swallow reflex may still be present. But the use of feeding tubes for extended periods also presents a set of risks. Atrophy of the same neuromuscular skills, and the possibility of infection in the stomach from the site of the gastrostomy (hole into the stomach). Nasopharengeal (up the nose, and down the throat) tube are generally only used in very short term cases, because of the trauma involved in the inevitable removal, and reinsertion that would occur with long term use.
One case I know of involves using the tube only for medications. The person has such a profound gag reflex, and such a strong emotional aversion to taking pills that she cannot (and will not) take her needed eleven different doses of drugs without the tube. The medications are essential to her continuing health. The tube was originally placed while she was unconscious. (recovering from an acute episode of the very problems her medication are meant to alleviate) The decision was to leave the tube in place, even after she had recovered enough to eat and drink normally, so it could allow her to stay on the appropriate doses of her medication. So far, it has been working quite well.
Most of the time, though, you don’t use tube feeding on a person unless it is necessary to avoid serious medical consequences. In those cases, just taking a sip of ordinary water is a very risky thing. A very capable, responsible person might be physically able to rinse their mouth, and spit, but for most people with serious disphagia, that’s pretty tough, too.
Tris