I like open source, but I _have_ to use mathematica to be as productive as my peers in my field. The open source tools that exist are indeed inferior and that's why I cannot use them.
However, this is not a good argument, as my field is physics. I do believe that the use of mathematica for science is a inherently bad thing, and should not be considered an allowed part of doing science -- because it is closed source.
I consider the reductionistic part of science very important, so when I use some function in mathematica I do not know exactly how it is implemented hence I cannot reduce my result beyond the point of "Wolfram says it is ok.". But that is not ok.
I have been lucky with all of my results, in the sense that one can check them by hand or by inferior products. But finding the result to begin with is why mathematica is almost necessary for a lot of calculations.