Ah, this does look like a more plausible reason for the Gemalto hack: http://www.theverge.com/2015/2/20/8079083/gemalto-sim-card-g...

Short version: Knowing the OTA key lets you push malware to the target phone SIM which you can use to surreptitiously exfiltrate data from the phone via SMS messages, amongst other things.