"We don't usually car camp. And we want to get to a point where we can take off for a month and travel either the CDT or AT."
There's no place on the CDT or AT that I would carry a bear canister, with the exception that in Glacier N.P. in June of last year the rangers made us borrow canisters because snow levels were so high that it put the bear wire too low relative to "ground level". That's not a usual situation, however. I use a bear canister in the Sierras (just did that in September), and on the Olympic coast, and would probably now carry one in the Olympic N.P. On the CDT it would not be a bad idea to carry one in GNP or Yosemite, though you're not required to. Basically, if in a place where bears are habituated (national parks and the like), take a canister. If not, don't --- they're heavy and they don't reduce in size as you eat down your food load. The "benefit" of being able to use it as a camp stool does not entice me in the slightest.
I think there is now a brief stretch of the AT that requires (?) bear canisters, but from my understanding if you can walk decent mileage per day you don't need to camp in that zone. There are a couple of places on the AT with bear poles or cable (Smokies, Shenendoah N.P., Katahdin area) but otherwise, perhaps apart from a short bit in New Jersey, really not much of an issue.
"If the canister isn't a good odor shield why wouldn't you want to keep it out of reach of bears? If it could break from dropping it seems it would break if a bear wanted in it. I couldn't likely handle having my food taken or destroyed if on a deep down the path adventure, especially if in the Rockies."
I think others have already responded pretty well to this, but just in case ---
The canister is very much NOT a "keep it out of reach of bears" strategy. It's not at all an easy item to keep out of their reach. If you want to use the "keep out of reach" strategy, learn how to bear bag, and hope you have adequate trees, a good throwing arm, enough daylight, patience, and a rich vocabulary of swear words. And then expect that you'll still sometimes do a poor job in keeping food out of the reach of an animal that evolved to climb trees (I'm talking black bears here).
Hanging it high and having it slip from the rope or other accident or bear activity could crack the can in a way that experience shows that bears don't apply. That said, my favorite place to put a bear can is sort of wedged in between downed logs, so that it's not easy to bat around and the thing it's batted against is softer wood rather than hard stone.