What are the Respective Costs of Christian Anti-Judaism and Exclusionist Messianic Teachings?

The Consequences of Christian Anti-Judaism

We know that Christianity is anti-Judaic.  There's the anti-Semitic Church history, the anti-Judaic hermeneutics which make Christians think that the New Testament doesn't promote Judaism but rather discourages it, the Christian preference for non-Biblical practices of pagan origin, etc.  So what is the cost of all of this?

It would be flat out wrong to say that Christians will suffer all the Deuteronomic curses because of this. The blessings/curses listed in Deuteronomy are primarily contingent on national obedience.  Thus, there's no guarantee that any individual person who observes the Torah will be blessed or that any individual person who rejects the Torah will suffer curses.  It's entirely possible that a Believer who lives his whole life unaware of Judaism could live a prosperous life--and even have a good relationship with G-d.  So why follow the Torah?

Because the Law helps us to know G-d, to orient to Him, to better our relationship with Him.  Thus, at a minimum, the Christian who rejects New Testament Judaism and opts for a syncretistic religion like Christianity is HURTING his relationship with G-d.  It means that a Christian's relationship with G-d is not as good as it could be.  

But, maximally, a Christian is distorting the gospel and this has really bad consequences for Jews who are in desperate need of hearing a gospel that the Torah will actually allow them to accept.

The Consequences of Messianic Exclusionism

Those One Law adherents out there know that Exclusionist Messianic Jews who teach that Gentile Believers in Yeshua are excluded from the national covenants of Israel (and the right/duty of all Israelite citizens to observe Torah), that this is not in accord with the teachings of the New Testament.  We know that the New Covenant is just as much a national covenant of Israel as the Old Covenant.  We see covenantal inclusion of gentiles in Acts 15 ("a people called by His name"); We see it in Cornelius, that it is praiseworthy (i.e. good) for a gentile and member of the New Covenant to choose to follow Torah (since he had a reputation for observing well the Torah of Moses);  We see it in Paul's instructions to the gentiles to abandon gentileness (1 Cor. 12:2; Eph 2; Eph 4) and embrace their status as members of the national covenants of Israel (Eph 2) and in his exhortation to the gentiles that they put into practice everything they've observed him practice;  We see it in Peter's statements to the gentiles (1 Peter 2);  We see it in the historical reality that ex-pagan Gentiles had no other religious option than Judaism and that they were well-known to be observing Judaic practices (Col. 2).  Etc, etc.

So what's the harm in those of the Exclusionist camp in teaching that gentiles are excluded from the national covenants of Israel (i.e. Old and New Covenants)?  

It would be wrong to say that such Exclusionists will suffer Deuteronomic curses.  However, if gentiles are meant to be included in the national covenants, then teaching them that they are excluded could, at a minimum, lead to role/identity confusion that damages their relationship with G-d, and, at a maximum, could imperil the souls of the ones who, seeking Judaism in non-Messianic communities, end up rejecting Yeshua in order to feel included in the Jewish community.