There again, Collins-Robert dictionary (5th edition, not the current one I know!) gives 'Je m'excuse' as 'I'm sorry', followed by one asterisk. The solitary asterisk denotes language used by all educated people in a relaxed situation but not suitable for formal occasions when you wish to impress. (For comparison, in English it gives one asterisk for phrases such as 'take it easy' or 'let's get cracking'.) I wouldn't want to be the one to deny the importance of colloquial language. I learned very formal French in England back in the sixties, and it took me a while to become familiar with everyday conversation when I went to live in France.