Somehow, I suspect that if kitniot weren't written down by the Ramah, no one would be keeping it today. That is, it is only textual Orthodoxy that insists on kitniot, not because of any compelling reason, but because the rulebook says so, stam.
Yesterday, someone was telling me of a friend whose child has asthma. The child's inhaler prescription lapsed, and so, when the child had an asthma attack, the childcare center refused to administer his inhaler (even though the prescription lapsed, the inhaler itself was still in their possession, obviously), even though his life was in danger. After all, the prescription has expired, and according to the book, they cannot administer the medicine!
My point is that it is no longer a minhag that people actually do (and by definition, a minhag is what people do), but rather, it is a minhag that people do against their will, because the book says so. Were it not for the minhag's being written down, I'm sure that people would have by now simply stopped doing it, naturally. But the minhag has ceased to be a practice-defined minhag, and has rather become a ontological halacha that is binding irrespective of what people do.
Al tosif. [Do not add onto the mitzvot.]
(For more on this general topic, see my thoughts at Minhag: haMakom or Avot?)
Someone else ("Shimon") there replied,
your comments remind me of Rav Eliezer Berkowitz's opinion that the codified halacha killed the torah shpb and the we became karaites of the shulchan aruch.
Shimon, in my view, you have just paid me an extraordinary compliment. I have been greatly influenced by Rav Berkovits's thought, and I have found that this has progressed to the point that quite unintentionally, I often will say things that are, in my view, reminiscent [in retrospect] of his thought. You here have confirmed my suspicion; I made no attempt here to imitate or follow Rav Berkovits, but apparently, I could not help myself.
According to Rabbi Chaim Shmulevitz, a rebbe has a talmid not when the student knows what the rebbe said, but rather, when the student knows what the rebbe would have said. Rabbi Shmulevits recounts a time when a student came to him excitedly to tell a certain hiddush (novel insight) he (the student) had had in Gemara. The student recounted his hiddush, and Rabbi Shmulevitz knew that he himself had given a lecture saying exactly what the student now was saying in his (the student's) own name! Now, reasoned Rabbi Shmulevitz, surely this student would not be so brazen as to tell one of Rabbi Shmulevitz's hiddushim to his (Rabbi Shmulevitz's) face in his (the student's) name, so Rabbi Shmulevitz instead reasoned that the student had learned Rabbi Shmulevitz's hiddush from him (Rabbi Shmulevitz), forgotten it, and conceived of it anew on his own. Rabbi Shmulevitz rejoiced, and considered it his happiest day ever; he had a talmid!
As an example of my unintentionally following Rav Berkovits, I'll note that my post Torat Hayim - The Torah of Life - EVOLVING Life began without any mention of Rav Berkovits; rather, I mentioned only Rabbi Avraham Shamma's teshuva on kol b'isha. It was only after I wrote the post that I realized that I had almost plagiarized Rav Berkovits, so I went back and inserted references to him, before posting. I also dedicated the title to him, as anyone familiar with his work will realize. Another of my posts, Does Modern Orthodoxy Have Any Legitimacy?, is also highly Berkovits-ian.