I could tell (= I understood) what you meant; using 'principal' here is a russicism though, a so-called 'false friend' :)
As to your question, I do think that having principles and sticking to them is really important for building healthy and effective relationships. In fact, being a person of integrity is vital in every sphere of our lives.
Your question is so good! I'm tempted to write an essay on it :) But I'm not doing it right now.
What is a "principle" for you? Are you talking about moral principles?
There's always the flip side to such things. What if someone does not share the same principles? You might clash on them with others, and then how can you tell that you're right and the other one is wrong? Whose principles are 'fair' and whose are not?
P.S. It's one of the greatest threads I've come across lately here :)
In my view, to be principled is to have a set of morals and beliefs which guides a person in every action and choice they make in life.
If a person would but take actions due to the purity of his intent, rather than for the incentive of a reward, or fear of punishment, he could be deemed truly, a principled individual.
The majority of us have principles of our own - some of these universally shared whilst others quite personal - but many of us however fail to observe these steadfastly. Instead, we waiver and compromise when situations are too difficult.