The problem is that it's hard to think like a packrafter until you've done a fair bit of packrafting.
The first, bare fact is that on most of the wilderness rivers in the lower 48 you'll usually match hiking speed, more or less. The exception is when the waters are high in spring and early summer, when you'll go a lot faster, but probably be cold and scared because of conditions. In late summer low water, or when running harder whitewater, you'll go a lot slower than the practiced ULer on the parallel trail up in the woods.
So why spend 1500 dollars to carry a bunch of extra stuff?
First, because you want to, or you wouldn't be asking.
Second, floating those big wilderness drainages is so much better than hiking them. The Bob is a perfect example. The trails along the Middle Fork Flathead and N and S Fork Sun have their virtues, but the South Fork Flathead trail sucks. It's hot and has mediocre views. In a packraft you just see everything better. I cannot overemphasize this point. After you've packrafted the Bob you'll view all other users as suckers, and you'll be correct. The Colorado Plateau is the same. Willow bashing and mud bogging on foot along the Esca or Dirty Devil is no fun. Floating is awesome.
Third, you'll look at routes differently, forever. This will spread out into other, non-packrafting pursuits.
Fourth, ordinary river trips become much easier. Want to do a mellow afternoon float with a friend/SO/kid? Pack two packs, drop a bike at the finish, and get it done. No need to wrangle friends, trailer, and a 200 pound raft. You can do a car shuttled packraft float trip for four adults with two Civics and nothing strapped on the roof.