A 3-night cruise is great for a first cruise or an extended weekend getaway. I've sailed three 3-night cruises (as well as longer cruises) and enjoyed each one. They tend to work better for my schedule because they can be sailed over a long weekend, whereas the 4-night requires a full week be taken off of work because they typically start on Mondays. If I'm taking a week off of work, I want to sail a 7-night, not a 4-night. So the 4-night cruises were never worth it to me for that reason.
Keep in mind that you asked this question on a cruise board filled with die-hard cruisers, so most here will advocate for a longer cruise every time. Some of them have never even sailed a 3-night cruise, and will put them down anyway. But each time I've cruised 3-nights, there have been lots of people onboard and everyone seems to be having fun, so I'm far from alone in my opinion that they're worthwhile.
The claim that "you only have time to unpack & pack & then you're off the ship" is silly and false. Unless it takes you a day and a half to unpack, and another day and a half to pack. Another bonus of a short cruise is that you can pack light.