It's a matter of battle readiness. How ready do you want to be? Would you rather have the skill to clear stoppages quickly and not need it or or need the skill and not have it?
See holdencm9's recent reply for a well-explicated explanation. I would add only this:
There are many things that are good to have. Getting those things requires the expenditure of resources and/or the acceptance of risk. It is not possible to acquire all good things, because of resource constraints or the acceptance of unacceptable levels of risk. So you have to choose what resources to allocate and what risks to accept.
Blindly accepting risk for something that one may very, very unlikely to need is unwise. I could "train" to learn how to rappel down cliff faces, on the theory that someday that might be my only avenue of escape from some rampaging horde. But given my actual lifestyle, that's so unlikely that accepting the risks inherent in learning how to rapel vastly outweigh the risks of remaining unskilled in that area. Now, I may decide that rapelling would be fun, in which case the analysis may shift. Or I might have chosen a different occupation for myself, in which case the analysis may shift dramatically. But sitting here today, "training" myself to rapel would not be rational.
In fact, training myself to rapel would be like taking out an insurance policy where the premiums exceed the coverage. It would be crazy. Training oneself to a fast TRB is not as crazy, but for many people it is like taking an insurance policy for an extraordinarily unlikely event with non-trivial premiums. Those resources might be better directed elsewhere.
Unless ingraining a really fast TRB is fun. Then the acceptance of risk - like the acceptance of risk that is inherent in going to a shooting range - may make a lot of sense.