The arguments about language efficiency on a single processor machine are probably outdated today. Most machines have multiple cores. We need good tools which can exploit this CPU architecture. Languages like C/C++ place a large amount of responsibility on the shoulders of a programmer. Effectively, you are writing two programs - one for the task at hand, and the other is memory allocation for it. Control does not always give speedup, also it is not always a boon. It implies managing the different resources on your own. In a large scale application, this is not a great idea. GC is a good alternative over Malloc and Free'ing when the size of the application gets huge.

Also, the scalability of a single system is limited. If you really need extra speed, I think going parallel is the key. Most numerical methods are parallelizable. And C/C++ were not created keeping this mind. It can be a programmer's nightmare to debug multiple threads with memory leaks. Languages like Java do make this task easier using programming paradigms like Map/Reduce.