It is a tough call - when I did the JMT in 2007 I went back-and-forth of whether to use the Ursack or not.
You are correct most issues with the Ursack was caused by user error and not cinching the knot correctly.
After much deliberation I decided to go with the Bearikade from Wild-Ideas. It ways 31oz and provides peace of mind that bears cannot get into it. It also provides a nice seat. While it is true that bears cannot get into a properly closed Ursack - they can trash the contents by jumping up-and-down on it and sinking their teeth into it and ruining the food from saliva (yeah a little gross). There is a metal insert you can additionally get for the Ursack to prevent this - however when I started adding up the weight of the Ursack, the odor back, the metal insert - the weight differences was about 8oz over the Bearikade and I decided just to opt for the known/approved solution.
Yes there are spots on the JMT that do not require bear cannisters - they do vary from year-to-year. You will find bear lockers in most spots after Muir ranch south.
I was stopped by a ranger in Evolution Valley and wanted to see my bear cannister. I started opening my pack and he saw the shiny lid of the Bearikade and that was good enough.
Note that I am no way associated with Wild Ideas - just a happy customer.
OregonBeerMan