1) Exploration of a given data set.
2) Automatic collection and analysis of data.
In most academic uses of data (1) is the case. Here, if the data is already rectangular, and in a nice format, I would actually prefer Stata. However, as soon as any sort of manipulation is required, or graphing something more difficult than a scatterplot is required I shift to R.
R really shines through when you need to do complicated analysis on a fixed data set. I've found that Hadely Wickham's `reshape` and `ggplot` packages are invaluable. They easily produce graphics that are more informative and better looking than any other graphics package I've seen. Additionally R has packages for essentially any statistical analysis that you could want to do.
While R is able to pull data from a database, or other places, as soon as you have more dynamic data, you enter into case (2). This is when it might make sense to start using python. But even then I've found python is mostly useful for curating the data so that it can be used by R.
R does have surprising capability for shell scripting and text processing, albeit slower than Python. I also use Python for the rapid text-processing necessary (possibly populating an SQL database) for R to eventually use.
I've been keeping an eye on SciPy, but there still seems to be a lot of "the source code is the documentation", whereas in R the documentation is usually superb and well-structured. And Matplotlib, while beautiful, seems to be more verbose than Matlab or R when it comes to customizing details of the graphics (e.g., axes, etc.). That's just my impression, but I wouldn't mind being shown otherwise.
Building a plotting library with the ease, sophistication, and beauty of ggplot2 in Python would be a big deal. A number of people I know are interested in venturing down that path (ggpy, anyone?).
While using two languages is an overhead, it feels like it plays to the strengths of both sides.
And since Python does web easily: http://rapache.net/
I'd be curious to see an updated comparison with LOESS added to the Python code!