Spray IS effective, regardless of what any prof would say. I've seen it work a dozen times, including on charging animals, and most any field worker will tell you the same. The key is the method of deployment.....and in that are limitations about how effective it is for riders. Spray is best used when a person sees the encounter developing and has time to prepare. For a surprise attack is has limited use because well teeth and claws and stuff. In the case you have a chance to make a break after an initial attack, and then prepare your spray, well okay. But for most surprise encounters it won't help. For deployment you have to know your product and have practiced how to retrieve your spray and remove the safety clip. Then you need to gauge wind because even 5kmh can reduce range by half. Then you are best to position an obstacle between you and the animal so you can deploy the spray from about 8-10 feet away. This is hard to do when riding. If you're in a group, well you have a better chance of course as one person can retrieve the spray while the other plays chew-toy.
Second, the canisters are somewhat delicate. Constant rattling can loosen the cap assembly from the canister, and result in accidental deployment. I've ultimately seen as many humans sprayed as bears....often by their own canister. Second, if you fall on it (as bikers sometimes fall), you also risk accidental deployment which can mess up your day. A cordura holster provides very limited protection. A hard container is great, but cumbersome and heavy, and takes time to open.
Your worst and most likely situations are generally surprise encounters. Thus, the points about making noise while riding bear country, are well taken. Avoiding known problem areas is also a good tip. Riding in groups is also very helpful, as predators generally avoid any type of group-encounter with animals/prey that is facing them down.
An accessible knife that can be carried without risk of self-stabbing makes good sense. It also provides something you can use to create distance for a moment, and then access spray if you choose to carry. However, a truly determined or fully enraged fully healthy animals is unlikely to be stopped unless you have the skills of a samurai. Statistics indicate people have better outcomes with spray over guns.....with some fudge in those stats as most people with guns are hunting, and are placing themselves in danger zones, or attracting hungry bears with their meat. Still, the reason spray works well is that you have to be prepared to use it, and be thinking in advance about deployment (create distance, find obstacle, gauge wind). However, it is a mistake to place confidence in a device, instead of preparation.
I have a tonne of respect of Sylvia Doulson and Get Bear Smart from Whistler. Her pamphlet on identifying and understanding bear species and behavior is great and easy to digest. It is really important to be able to know what you are dealing with in terms of species (black vs Grizz), and in terms of the type of encounter (surprise, defensive, predatory etc). Reading the situation is critical to choosing the right response. Education is more important than any device we can carry.