Final printed materials don't need to be more than 300dpi. Consider, for instance that modern retina displays are around 260 dpi - you're eye can't distinguish much more than that at a normal viewing distance.
That said, the files you are working with should probably be larger canvas and/or higher dpi so that you can enlarge, crop, focus, etc. on sub-pieces of the artwork and up still above 300 dpi. If you made card art at 2.5" x 3.5" at 300 dpi and wanted to zoom in on a part of the artwork for final layout, the resulting image would be less than 300 dpi, which isn't good.
I actually work all of my card designs in vector and can export the files at any resolution. 300 dpi cards come out to 750x1050, but I get the actual art made at a minimum of 1080x1512 for resizing. A lot of my artists create their stuff at much higher resolutions, which is great for wallpapers and stuff.
I just received a response from my manufacturer who said all print materials need to be 300dpi. Said 600 was overkill and made the files unnecessarily large. Looks like the industry agrees with you guys.