A few problems with it:
- It's inhumane. It involved mass divorce, ripping husbands and wives apart, and tearing children away from their fathers because the kids weren't "pure enough." Whether the wives and children made the journey from Babylon or were local non-Jewish women, the result was the same: dishonor, shame, and a life of misery. The "extreme circumstances call for extreme measures" line is used to justify all manner of atrocities, and I'm unconvinced that more humane measures weren't possible.
- Jesus flat-out says that to divorce a spouse who has not been unfaithful is to commit adultery. Paul adds an allowance for divorce when one partner has converted and the other can't stand it, but in that case it's a mutually agreed-upon release from an unpleasant situation, not a racially driven purge.
- It's inconsistent with obvious cases like Ruth and Rahab, where pagan women were accorded places of honor in Israel and Judah's histories.
- It's also inconsistent with the scriptural principle of honoring your vow for the vow's sake, even if you realize it was a vow you were unwise to have made in the first place.
Lastly lastly, I really hate the idea that women are the source of evil and destruction in Israel and Judah. Patriarchy sucks.
What I'm hitting on here is one of the thematic struggles of the Tanakh, whose authors sometimes had sharply differing views of how to treat Gentiles. I believe it's the Priestly writer who warns sharply about the dangers of associating with Gentiles, tells the people that they can enslave foreigners in their midst, and so on; while the Deuteronomist tells the reader to show kindness to the foreigner in their midst, for the Israelites themselves were once aliens and strangers in a foreign land, and reminds them to leave grain on the ground for the needy and the alien. Ezra says "Get rid of these foreign women"; the book of Ruth says "Let them in, so they can experience the wonders of our God and our people."
On the one hand you have the order to keep separate from the pagan nations surrounding them -- something particularly hammered home in the books written post-Exile -- yet with the other hand, you have writers like 2 Isaiah saying that all the nations will come to Israel and worship the Lord on his holy mountain.
I've heard the "keep them pure, keep them separate" argument before, but it doesn't match up with vast other parts of the Tanakh, where Israel is meant to be a blessing to the nations, where the Queen of Sheba comes to Solomon and leaves amazed at the great wisdom the Lord has given to him, where the nation is uniquely positioned to spread news of the One God throughout the ancient world.
Did the command to evangelize the world come with Christ, or was it in place thousands of years earlier?