And so we're going to delve into this a bit. First, let's lay out some approaches to "halacha":
Christian approach: we don't need rules; we've got the Spirit to guide us.
Anti-Rabbinic Messianic: we don't need rules; we've got the Spirit to guide us. Here's the example of David Stern (which feebly tries to differentiate itself from the Christian approach but ultimately ends up expressing disdain for rules):
"All of this is by way of background to considering what the role of halakhah might be in the light of the New Covenant. According to the New Testament, every believer has in him the Spirit of God....It is clear that people need guidance...But Christian [worldview] tends to float above specific rules...to a [world] of general principles...[which is] inspiring but far from the brass tacks. Messianic halakhah can provide specific guidance for those who seek it. It can provide a basis for discussion, for probing the direction of finding godly solutions to ethical questions, as well as for ceremonial situations, helping to establish communal norms--norms, not hard-and-fast rules!" pgs 153-154 of Messianic Jewish Manifesto by David Stern.
Pro-Rabbinic Messianic: we need rules; otherwise, our communities will disintegrate into lawlessness. We should be "righteous before God, walking [i.e. practicing halachah] in all the commandments and ordinances of the Lord blameless," (Luke 1:6). We must be followers of the Way (Acts 22:4) not followers of the Ways--there must be logical consistency because our G-d is logically consistent. We must "walk" as Paul walked ("Brothers, join in imitating me, and keey your eyes on those who walk according to the example you have in us" Phil. 3:17).
Now let's return to Chaim Saiman and his story about the rabbi who walked into a church. He was trying to find out how the Christians operationalize the command to be fruitful and multiply:
"I then paused, saying, 'Let's leave the rabbinic answers to these questions for now, but let me hear how in your tradition, you reason towards the answers. Surely you want to fulfill the word of God, so how do you know when you have done it?'
The answers clustered into a few categories. Some thought that it was not a command to individuals as much as a charge to society as a whole, such that the verse did not direct individuals to undertake specific acts. The majority, however, responded with something like, 'as many as you can handle,' 'read the Bible and let the holy spirit guide/inspire you to the correct answer' or 'discuss it with a pastor and other members of the faith community in order to reach the conclusion that is right for you.'....Repeatedly, the churchgoers told me that by focusing on the law rather than the heart, by not allowing faith to guide the answers and by legalizing what should be an intensely spiritual experience, the Talmud completely misses the point.
Stepping back, it became clear that I was watching radically different methods of Biblical and textual interpretation at work. The Mishna and Talmud, and in their wake rabbinic Judaism, conceptualize each of these questions as inherently legal. The reasoning process involves (to use the modern lawyer's terminology), reading statutory language, examining relevant case law, identifying the latent ambiguities and employing conventional forms of legal analysis to arrive at a conclusion...there is little in the rabbinic process that surprises the classically-trained legal mind.
The perspective I heard at the church that morning was very different. Very little of it resembled 'law' in the way lawyers would use the term. In fact, many in the group felt that supplanting the letter with the spirit was the central goal of Jesus' ministry. Jesus spoke to the heart. [Christians believe] that Jesus resisted the Pharisees' impulse to refract the religious experience through the prism of law. [Christian aversion to Talmudism] expresses a theological commitment embedded deep within the Christian worldview.
The divisions between the approaches can be usefully, if somewhat crudely, articulated in the terms of contemporary legal thought. The rabbinic view tends towards solving these problems via the application and analysis of rules, while the church group approach tended to resist rules and favored the application of broader, less fact-specific standards. Moreover, on its own account, halakhic reasoning is understood to be objective. It involves the application of texts and precedents to facts, and at least in theory is unrelated to the faith or spiritual temperament of either the questioner or the rabbi charged with answering him. Finally, and most broadly, the Talmudic mode assumes that any issue relevant to religious, social or economic life is both justiciable and answerable within the normative bounds of the halakhic-legal framework.
The Christian (particularly the contemporary Protestant) mode inhabits a very different discursive realm. Law is not the relevant platform through which to analyze and decide important religious and social issues. It is thought to be overly restrictive, and unjustifiably replaces faith and love with rules and precedents. Instead, the reasoning process is directed inward, and exhibits more overtly religious, spiritual and subjective modes of reasoning. While a discourse, premised on seeking inspiration from prayer, Bible reading, and conversations within the fellowship, may produce fewer specific conversations guidelines, the believer readily trades rabbinic legalism for a method that actively engages the hearts and souls of the faithful," from "Jesus' Legal Theory--A Rabbinic Reading" by Chaim Saiman, Journal of Law and Religion Vol. 23, No. 1, 2007/2008.
Does having the Spirit mean we must abandon the reasoning process associated with Law? Does having rules mean that we have rejected the Spirit?
Or could it be that rejecting rules leads to self-deception, self-legislation (i.e. lawlessness), and ultimately self-destruction?
Does G-d command us to put our heart before our head? Or our head before our heart? Remember that the heart is desperately wicked and will tell you "Well, Messianic halacha is vague anyway so I don't need to keep Shabbat" and "Well, I don't feel led by the Spirit to do anything at the moment so I'm not gonna keep any of those Pharisaical rules."