I don't think your typical INFP is a long-term planner either, but I don't think most of is view commitment as planning.
I don't like to commit to many long-term things because I want to stay open to different possibilites as they emerge. I don't like to be limited, and I like when life surprises me. INFPs are rarely ever control freaks, for that reason. We share the IxxP attitude of focusing on refining our inner model as a sort of compass and then being ready to improvise and adapt as needed to the external, dynamic world.
When I think about the future, I think about my internal self...what I would like to feel. This usually means being aligned with big concepts also, such as experiencing creativity, autonomy, spirituality, intimacy, etc. I am very open to the many ways these things can manifest. My idea of them is constantly being refined, as well. But I also realize some commitment is involved for the development of higher goals, even if there is openness to how they are realized. What I usually just want to see is that someone is not going to confine me too much, that we both can grow, but also that there is some stability and tie in the shared feeling.
So an issue for me (and likely most INFPs) with relationships is loyalty. I know things change, but I want to feel that someone will not take off the moment something gets difficult, which is inevitable in life. I am a whole, complex him with flaws, weaknesses, bad moods, bad days, bad weeks, etc. I don't want to have to be a cardboard cutout, a Stepford wife, a Geisha-bot, etc. I am willing to accept the whole of someone else also. Of course, the good should outweigh the bad.
Sometimes the "take it day by day" types of people leave me with the impression that if any difficulty ever arises, then they are out; as if, the history or larger context doesn't mean anything at all, and neither does the person (just what you "get" from them). It seems there is no loyalty there, to me. To me, the pleasant times are not static, but neither are the difficult ones. I cannot define things by a moment then, but instead, a larger, continuous dynamic.
If someone cannot see the big picture, then I am left feeling on eggshells, and then a real intimacy is never going to happen. I also cannot call that love, which I do not think is so fickle or so selfish.
So, I think it is reasonable for people to desire commitment, as a kind of pledge in good faith, so that there is enough security to be vulnerable and cultivate intimacy. I dont think that means life is planned out far into the future.
The way ISTPs are described sounds fickle, and frankly, a bit shallow and self-serving, although I don't necessarily see them that way in person.