I don't disagree that it is a limitation, but I don't see it as the biggest problem with backup. Personally, I see more value in the protection of OneCare - virus/spyware, firewall, tune-up. As for the expense, it's quite reasonable for 3 PCs, though a bit expensive if you pay full retail for a single PC.
You might want to look at some free backup solutions. Personally, I like SyncToy to keep folders in sync between multiple PCs and also use it to sync a few folders between drives. There's no compression, but it works quite well.
Download details: SyncToy v1.4
-steve