As most of you know, Judaism has a number of internal conflicts—for example, conflicts over who qualifies as Jewish. The Messianic movement frequently inserts itself in these conflicts in an attempt to gain legitimacy. Here’s a great example from Yahoo! Answers:
THOUSANDS UPON THOUSANDS of modern Jewish converts are repudiated, rejected, and considered merely “righteous gentiles” who are playing at being Jewish by Orthodox Jews who do not accept their conversion status—-why should Messianic Jews be left out? Of course they’re not accepted—-just like Conservative Jews, Reform Jews, and “Reconstructionist” Jews are not accepted.
Of course, there’s a world of difference. The Orthodox might not recognize converts from the Conservative, Reform, and Reconstructionist movements, but those converts were accepted into a normative Jewish community. With Messianics, however, you have a group of people showing up uninvited and arbitrarily declaring themselves Jewish—oh, yeah, and declaring that they should be allowed to keep their Christian theology and evangelize to Jews at the same time.