I get what you are saying, and I have several points to make here.
First, there is a difference between a contract and a pledge. A contract is something I sign
voluntarily with the other side, which legally states what each of us is to do, and what happens if one of us fails to uphold the contract. A pledge is something you say, but it does not bind you to anything legally in itself. From a contract, you get certain benefits if you uphold it. Pledge gives you no benefits, it is a purely self-restricting act.
Second, of course a pledge can influence your decisions; the question is, whether it influences them in a positive or a negative way. Who is to say that your decision to stay in the army was the right one? Since it was mainly based on an abstract set of phrases you've said, rather than on something tangible, logical, pragmatic, you were basically rolling a dice and accepting the outcome.
Finally, like I said, everyone is free to make any pledges they want. One should not be
required to give a pledge, however. There is a legal contract that prescribes what you should do in a given relationship with another (others) human being(s), and beyond that everything should be solely up to you.
Bottom line is, you don't need any pledges to make reasonable decisions. If you are happy with something, you don't need any pledge to keep being happy with it. If you are unhappy with something, then a pledge will only be an unnecessary hindrance. If there are positive and negative sides to something and you are having a hard time deciding (as was your case), then a pledge won't give you any new information helping with your decision, and
if you make the decision based on the pledge, then I wouldn't call it a quality decision - even if it leads to the better outcome. You might as well, again, roll a dice and accept the outcome.