Agreed, the MFF SIM makes for more reliable hardware. But surface-mount assembly is permanent. When you buy the SIM from one particular network operator, like Deutsche Telekom, you are locked in.
In theory, it should be possible to buy a 'blank' SIM, embed it in your device, and then program it with the network access credentials for any Mobile Network Operator. But as usual, commercial paranoia means that this will probably never be allowed.