This "one thing it does well" business is then presented as : "using the right tool for the right job" and it's difficult to argue against that because the counterpart can easily deride you as a fanatic of some technology, someone not objective enough, etc...
It is however interesting that we used relational databases for virtually everything for decades even though SQL is suboptimal at most things if we take them in isolation. Some will argue that people are now realizing their mistake, but the truth is these companies were successful and we were all getting our paychecks. (PS: I choose to use NoSQL for virtually all my projects)
The real driver shouldn't be the one thing it does well. Many times - if not most of the time - it's preferable to use a tool optimal for the most important parts and suboptimal for the rest. I personally prefer to provision two more instances, than to add two more technology stacks.