Even if it has a door, which will limit the dust on the instrument?
The wood of a guitar will swell and shrink in response to changes in humidity and temperature. Unless the instrument is subject to extremes, very hot or very humid/dry, these changes do not affect the instrument other than putting it out of tune.
A cabinet, especially one that seals well, can help keep the instrument more stable in regards to the temperature and humidity, but unless your house has extreme variation it isn't really necessary.
The cabinet can also help protect the instrument from accidental damage, of things like small children visiting...