It really depends on your application domain. Someone knowing that the complex thing I want to do is already in NumPy and she used it herself yesterday and here's a snippet of code pasted into my IM is priceless. Similarly I might say, oh, I wrote some code that does nearly your thing, you could just change this one line. Pretty much anything I need I could code up myself in a half a day or a day, but all those half days add up.

More than one project I've started in Lisp here (not to pick on Lisp, just as an example) and run into an issue that I could implement myself in time T or do the whole lot in Python in time T/10. It's a no-brainer. For non-trivial tasks to be completed within fixed periods, striking out on your own with a new language is a meaningless risk.