that even I, in my limited understand, can easily perceive:

1) First and foremost, as a demonstration to His people that it is impossible to achieve sinlessness through human effort, because there is no man (save Jesus) who could obey the law fully in spirit and in action. The law shows us the need of His mercy, and of Jesus, our savior.

2) To provide a set of instructions to His people that, if followed, would virtually guarantee they would become a strong, healthy and prosperous people. So many of the laws of living involve principles of health and cleanliness that were absolutely necessary for the time in which they were written. Does that mean they should now be disregarded. Not in my opinion. There are subtleties to His laws that we perhaps cannot comprehend which, if followed, will reap benefits. The subject of menstrual coitus could very well be one of them.

An example: The Torah laws regarding coitus were very clear. If followed, a husband would not have relations with his wife until 14 days into her cycle. At that time, relations could be resumed. Guess what the result of following that law would be? It would produce more sons than daughters, since 14 days is day one of the ovulation cycle for a majority of women, and the little male-bearing sperm swim faster than the female. The little male swimmers are going to get there first and find a fresh, healthy egg to make a new little Jewish boy. Why would God give that instruction? His fledgling nation had need of warriors, and following this particular law would provide warriors a-plenty.

As far as whether the law should be obeyed? This says it perfectly:

guess you will have to argue with Yahshua about what He means by...

Mat 5:18For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the TORAH, till all be fulfilled.
In Paul's preaching to the Gentiles and excusing them from the law of Torah, he eased the way for salvation of the Gentiles. But if one were to wish to maximize both the relationship with God and his personal benefits to us, IMO, the right way would be to become a messianic Jew.