Is there a reason for this restriction or is this a bug or a new feature request to have it changed?
I expect SMS messages to be sent to a contact if at least one of their phone numbers is marked as a mobile phone number, even if it is not their primary phone. I suppose it might be confusing if they had more than one mobile number, but picking the first of several mobile numbers would seem preferable to simply not sending a message if no mobile number is picked.
Also - this restriction seems limited to sending SMS as an activity. I can still send a Mass SMS to contacts even if their primary number is not a mobile number.