I'm not sure if this has been brought up yet, but there is another important reason for an SD card: getting your data off of the device when it inevitably fails. Data saved to onboard memory cannot be retrieved. Data on an SD card can be retrieved. Of course, you can use network backups to get around that. But network backups won't be as up to date as the actual saved data for most of us.

As for durability: I suspect that the uSD card slot is more prone to failure than an SD card slot, while the cards are just as durable. (Smaller mechanisms for ejecting. Smaller contacts holding the slot to the board -- and they are more likely to be surface mount only without through-hole posts.)