I just upgraded to Tiger and discovered that I can only isync with my other computer via .mac!
Now I am forced to subscribe to .mac if I want to sync computers, and my .mac volume will increase.
On topic: I believe iSync is used so you can sync devices like cellphones, PDAs, .Mac, etc. not other computers. I'm sure there are 3rd party apps for syncing computers, and remember that iSync syncs contacts and calendars mainly. A quick versiontracker search gave me this.
It would be nice if Apple gave .Mac memberships to all mac users, but I can understand the cost to them. When it started it was free, I can only assume they started to realise the costs!
I use Chronosync to sync Documents between my two macs, works great. However, for calendars and contacts I use .mac to keep them the same on both Macs.