Polyglot approach involves picking the 'best' language for each micro-task and then trying to get them all to interact and play nicely to each other. The more 'normal' approach is to pick 1-3 languages that are "good enough" at all the necessary tasks.
But then wouldn't this require a bigger team with vast expertise or a lot of time compared to a more normal approach, and it will only result in marginal performance improvement, which might not be important in most of the regular projects?
"Best language for a task" isn't meant to refer to performance of code (except secondarily when the task is highly performance sensitive) but rather the language in which it is easiest to accomplish the task.
Using the right language (and technology say, NoSQL vs SQL) for the job: polyglot shouldn't differ from normal software development - it should be the norm. No one language is the right tool for all tasks.
The counterargument is that each time you add a new language to a project you add complexity, maintenance overhead and interop. problems. Sometimes a good enough tool is good enough and 'buying' the best tool is too expensive to be worth it.
We're not talking about using a tool for something that it wasn't designed to do, rather using a tool for something it was designed to do, but which might still only be second or third best tool for your particular job.
Calling a Common Lisp function from your Rail project might not be the best solution to your problem all things considered even if the Ruby version of that function is both slower and less elegant than the CL version.
In that case it comes down to economics, specifically: do the benefits of using a second or third best tool outweigh the costs of using a tool that is not the ideal. Or would the benefits conferred by using the best tool outweigh the costs.
It doesn't come down to it: it was about economics all along. All these philosophies -- polyglot, more monoglot, etc. -- have costs. You can choose to ignore it sometimes by staunchly recommending one approach over another in generic settings. But the underlying costs are still there.