Ah, I see. You are not disagreeing that the use of "personally" in those examples is a pleonasm. You are stating that you believe using it suggests you are not trying to impose it on others whereas I believe it makes the speaker sound a little pompous and that they believe because it it their opinion it carries more weight.
Different opinions, both perfectly valid.
Re your examples:
Shoplifters should get a life sentence.
This is being stated as fact, rather than opinion. Unlike opinions, individuals do not get to have their own facts.
I think shoplifters should get a life sentence.
I agree with your analysis of this one.
In my personal opinion, shoplifters should get a life sentence.
I agree with your second assessment "I think I'll sound posher/smarter/more important/more formal if I add more words." However, I also agree that people may believe they are cushioning it but not realise how it comes across to some of us. Equally you could argue that if I said "In my opinion, shoplifters should get a life sentence", I am pushing my opinion as valid.
Re But I disagree with your opinion that people should always stick to what's "sufficient".
I don't think that. There are times when a bit of wooliness and extra emphasis is helpful. i just don't think that is the case with the use of personal/personally in many of the instances in which they are used.
Re the professional/medical/legal opinion, they are still the personal professional/medical/legal opinion of the individual. That is why we are frequently told to get a second opinion.
The pleonasm is between "my" and "personal" or "I" and "personally". The word opinion is irrelevant to it.
But the great thing about opinions is that I respect your right to have your opinion on whatever you like. I also know that me disagreeing with you or you disagreeing with me doesn't invalidate either of our opinions. It simply means we have different opinions.