It's not idea, since it makes your script a lot less portable. A native python script can run on any unix or windows machine that has the proper python libraries installed. When you add shell commands into the mix, you break that, and suddenly are locked down to a much narrower subset.
Sometimes you don't have a choice, but if it's something as simple as that, writing the code natively in python would make a lot more sense, and also be faster to boot (since the python process won't have to spawn a new shell just to execute the one command).
