Well, frankly, you don't.
Its nice to have it when troubleshooting things, or installing Win on your Mac (for choosing OS X before installing Bootcamp drivers), but you can always keep GT7300 near for those things.
Some cards (AMD) have minor bugs if used unflashed but its rare (although i had it with HD6850 in FCPX).
Some people need PCI info to be functional for various cards they're using, also not possible with unfleshed card (it shows blank/error).
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And having so much topics about GTX680, 7950/90 and R9 280(X) flashing its fairly easy to do.
Again, if you don't need it than the choice for GPU upgrade on MP1,1 is even easier - almost any card will work.
EDIT:
AFAIK, yes, but you'll need OS X 10.10 as minimum for Maxwell GPUs.