There are a few guys on my son's d3 team that are currently in the "3" part of the 3-2. One transferred to do the "2" part.
The advantage is that you get a much more well rounded education. I'm in a huge engineering town and have been told by over a dozen engineers in management at major firms that they desperately need engineers with good writing and speaking skills, good people skills and the ability to work in groups. Most good liberal arts schools emphasize this. Most engineering programs don't.
The reason: engineering firms have to present their products and explain their projects to government and business officials who don't speak engineer. Most engineers are math and science guys with poor personal skills, poor writing skills and are terrified of speaking in large groups. The standing joke here is that the definition of an outgoing engineer is one who looks at your shoes while he talks to you. The disconnect, loses business and government contracts.
The disadvantage, a certain 5 years of college tuition (possibly 6), and will your son really want to transfer during his senior year?