I'm not in the market for a tablet at the moment, but this would be a deal breaker if I was. My reasoning is a bit different though, since I don't stuff tonnes of data onto my tablet.
Reason 1: sometimes I swap cards between devices. If my phone has something that my tablet doesn't, it's far easier to swap cards than it is to hook things up to a computer or setup a WiFi connection.
Reason 2: I often break things down according to function. My reference library is on one card, teaching library on another card, and recreational reading is on a third card. I find that this helps me to maintain focus while reading.
Reason 3: it allows the tablet to be used as a card reader. I've bought card readers before, and they always seem to break. Not sure why. It has happened to cheap readers and expensive readers. Most of the time it seems to be a problem with the electronics. Never had a problem with the readers built into my phones, tablets, and cameras though.
Reason 4: I tend to use devices for unintended purposes. Removable media makes this easier, especially if the system software is on removable media (thank-you Kobo!)