As a member of an alliance between nations, the United States should have an obligation to defend all NATO members. When entering into international agreements between nations, such as NATO, all members must be aware and accepting of the nature of an alliance, and be willing to step up and help defend other members in times of need and crisis.
Though I suspect NATO has outlasted its original purpose, if the US wants to pretend it's part of the NATO alliance, it needs to fulfill the mutual defense portion of that agreement. If we don't want to help protect the other countries in the alliance, we should pull out of it completely. There is a reasonable argument to be made that the other nations wouldn't be as quick to assist the US as the US is to assist them, but that's the hazard of being a "superpower."
Keeping promises is an important part of being a world power and a world leader. The US counts on other countries to keep the promises they have made to them, and if they go back on their word, the US is no longer trust worthy an there is no reason for other countries to keep their promises.
Yes, because the United States is part of the NATO alliance, it should do what it can to help fellow NATO members. This is how the alliance works. Everyone in the group needs to do its part and help as it is able. No one nation, not even the United States, should be expected to do everything for everyone, but each country should do its part.