Fiddlerman said
I understand. It's OK for your thumb to be adjacent to some of your other fingers but when and if you start shifting past 3th position in the future, you may have trouble moving your thumb back and forth. If it works well for you, I would keep doing it regardless. I've seen players who do this and still play wonderfully.  

I just recalled my former teacher grabbing my thumb and pulling it like, "Put it there!" She wanted it to be flexible instead of fixed, but she expected it way too categorically. I found it brutal, it made my feel bad and I couldn't relax at all like she expected me to. Sometimes it even hurt if she was so rough. I knew she was wrong but I also knew the relationship between me and her would have suffered if I had acted out the furious leopard inside me.

I think it's enough to be reminded now and then, that the thumb should always strive for the most comfortable position and that can mean changing its position while playing a scale. Yes, I often look at it like, "Are you all right little man?" Because I do feel that the 3rd finger suffers most if the thumb is fixed. Especially then the progression of a whole note step is more tiring.

Fiddlerman said

Demoiselle said
I just recalled my former teacher grabbing my thumb and pulling it like, "Put it there!".........

At least she didn't hit your fingers with a ruler while you were playing. LOL

Honestly, that sounds terrible. I condemn her actions!  

I know I'm a fanatic too if it comes to my 1600s topics, but her issue was basically fanaticism which showed in many side aspects. When I guessed I might have lost a little piece of paper weeks ago, she looked very angry for a moment because she's extremely tidy-minded. She deeply hated tuning devices because Pythagorean fifths were like divine law to her and any different system would start a very long debate (on my money). And if she felt like my knuckles weren't flat enough she kept pressing them down forceably. I did learn the basic techniques like bow grip and finger legato, but in the end she was too focused on her personal experiences to really solve my problems. Like again and again suggesting her beloved 3 little wooden balls to make the hand soft as she said. And as she forgets soon all these things repeat like forever. She's sort of mentally trapped and needs help. But mostly she has a very nice attitude, unlike a thumb-pulling bully. It takes time to learn about the 'demon' inside her.

You hear that from many people: issues with violin teachers, parents complaining about the teacher of their kid. We easily bump into a teacher with serious issues which don't always show in the first lesson. The violin is a lovely instrument but it is merciless and can seriously hurt people. I wonder how many professional violin players are out there who have been hurt by their instrument at a very young age? Actually it's not the fault of the instrument but our high expectations and adoration of this queen of instruments.