In the event of Scottish independence, the Scottish economy will be in the toilet. In this event, Britain should not be allowed to become a magnet for Scottish economic migrants.
I wouldn't blame Cameron if the Scots choose independence; the Tory party don't exactly have a strong foothold in those parts. However, I will certainly excoriate him if he gives an inch in any negotiations in the event of independence - especially if he allows large scale unskilled immigration from Scotland into Britain.
An independent Scotland would have almost zero negotiating power and Britain should exploit that to maximise our own advantage.