I just joined the adapter-game...
Normally, when developing software from an installed Linux, you'd write your compiled binary to the SD card, eject the SD card, move it to your PC or Mac, then copy the file, remove the SD card, insert another one, copy the file onto the new card, eject the card, insert the card in Raspberry Pi and reboot...
I think this would only be if you did full builds on the Raspi. I'm imagining just building a cross-compile toolkit on my linux machine. Either method would work though.
If you want a second SD card adapter...
...You could write the file directly onto that SD-card, eject both cards and insert the new card into the 'master' SD slot and reboot.
I don't think (at least for my application) a second SD card is really required. I already have a USB SD reader, which can be easily plugged into any of the six USB ports I plan on putting out the front. (the 7th I plan on pulling power for the Raspi from)
I just found a panel-mount USB cable too:
But I think all these adapters are way too expensive. Must be possible to get them cheaper on eBay.
I think just putting the USB hub in place so the ports face out will be enough. Your spot on (about it being too much) if I where to use six of these things for my USB ports out. I think more then I want to spend, but I will still think about it.
Apart from the above, you might want to consider having a usb-to-serial too; you can find $1 adapters on eBay, but they require a driver (probably prolific PL2303)
We use these things at work sometime. I cannot think why I would want one in this case. At the vary least, the usage would be only occasional. I think attaching the dongle for those times to the outside-facing USB ports would be fine.