As a user of the device, I can come up with a few reasons to upgrade:
1. No drivers -- it's a USB Human Interface Device that can be interacted with directly from a user program. The HS22 drivers are buggy and are not run through the Microsoft testing laboratory (i.e. they are not signed so you have to click the "Yes, I know I'm doing something I shouldn't box during the install"). The HA22 drivers will bluscreen some systems when you exit the user program. The "latest" HA22 drivers available are at least 2 years old.
2. Better radio antenna -- I can't prove it cause due to driver issues I didn't use the HA22 that much.
3. 40kbps data rate vs the slower data rate that is found on the HA22 (I think).
4. Support. Intermatic appeared clueless when I tried to get them to provide working drivers. First, their "support guy" was unavailable. After a week, I aske again and they said they were "testing" new drivers. I gave up on email and called them. Later that day they supplied "new" drivers that were supposet to fix things but were just repackaging of the same broken drivers.
5. Thin -- you can use the other USB ports nearby cause you don't have a big thing in the way. you probably don't need a USB cable unless you've got something like a PC that's under a metal desk.
6. Installer firmware that supports ThinkEssentials Pro.
Steveg