If you are camping etc. it's probably worthwhile otherwise pay as you play. For the Icefields Parkway from Lake Louise to Jasper it is a must-pay. On the other hand if you are simply driving Highway 1 from Revelstoke to Calgary (through several national parks) you don't need to buy a pass - unless you want to stop - which is highly recommended!
On the third hand an annual pass will be good fro 12 months from the month you buy it, so you can come back!
Basically, at $9.80 per day, you've got to stay more than 7 days in the parks to recover your investment. Camping is extra no matter what. Since I'll only be in Canada at most 6 days, I'll just pay as I go.