Generally speaking, violating positional elements, at any time, is bad for positional understanding. When you win a lot of games with bad positional play, this provides positive reinforcement for playing badly.
This opening, often called the Black Bear of the Philidor, isn't really breaking the opening principles. Black has gained space on the kingside, ...
I'd say the opposite is true. "Principles" are great as general rules, but if chess were about following a set of fixed rules, you could simply buy a book that contains those rules and become a Grandmaster after you've finished it.
But that's definitely not the case. Chess is a game of exceptions. Learning about this opening will help you find ...
FM Andrey Terekhov's Two Knights Defense repertoire on Chessable uses this variation against 4.d3.
That repertoire deserves to be more widely known: it's one of the best on Chessable, the author updates it frequently, and it's free!
In the introductory text of 4.d3 h6 he notes:
In the beginning, this line has been mostly used as a surprise weapon, but in ...
Probably I did not fully understood the question but if you just violate principles, either:
you are on your way on learning them so you should follow them, use
and understand them to know when its OK to break those
or you already know them but you have your reasons (like confusing
lower rated opponent, or surprise someone in a blitz game, etc.) to
White has a development advantage with an extra piece in the field and therefore should be able to capture some type of initiative despite the symmetrical pawn structure.
I suggest playing through some top games in the line to see how things play out.
White's position looks pretty active. It's basically a zero-risk opening that offers many chances to play for a win. For example, White can continue with a quick Nc3 (speculating with Nd5 ideas that will give the d8 bishop a hard time to join the game again), Be3 and 0-0-0