The benefit for a guy like Longoria (or Swanson) in signing a deal like this is they are guaranteed their first fortune at a very young age...years before they would have been had they gone through arbitration. Some people would rather have $30M now vs $40M in 3 years if they play well and stay healthy.
I think offering him a Longoria-type deal when you call him up also puts you in a really good position to keep him for his whole career. At 31, there's likely much less chance someone comes along and offers him a 7+ year deal at market value. He'd be more lined up with the back side of Chipper's career at that point. Assuming he is what we think he is, the organization would probably be in position to sign (or extend) him at a bit less than market value through his age 35 season - by 2025, that's probably $20 million per - so maybe you get those years for $17-$18 million each. At that point, both sides would probably feel comfortable going year-to-year as long as Swanson wanted to play and was productive.