I work in Visual Effects for film and TV. Aside for the apps we use almost nothing people regularly use on a mac will push you to need a 4500, but I am very glad that apple offer them as it shows a commitment to high-end workstation graphics. About the only thing I can think of that I have used a mac pro with a fx4500 was Maya 8, this in itself is mildly spurious as it was runs under rosetta. Apple have bought Silicon Colour and some other apps which is a very good sign. Most high end Discreet systems such as Flame/Inferno and Lustre now rely on the same hardware as the top specc'ed mac pro. That is Dual Woodcrest, FB Ram and Quadro 4500's
For creating a finishing station such as a Flame or Smoke a 4500 is essential as it provides the necessary openGL throughput. if Apple chooses to evolve their Pro apps into this market (which many of us have been begging for !) then the 4500 is a necessary piece of hardware.
Similarly now that Maya is offering 8.5 as Universal Binary, I think that Maya on a Mac is potentially a viable option. Similarly for things like Shake, After Effects and Motion, Cinema 4d and whatever else you really learn to appreciate workstation class graphics cards. It's not really about performance anyway, it's MUCH MORE about stability, features and qualified drivers.
I didn't buy a 4500 for my Mac Pro at home but one of the machines I work on has one !