Giving him asylum in the embassy or exfiltrating him to Israel, particularly in such a public manner, would have risked escalating what was already an embarrassing diplomatic crisis, and would have represented an immediate and public admission that Pollard was an Israeli agent. To the Americans, Pollard was a federal fugitive and a traitor, and giving him asylum would have been seen as a provocation, with potentially serious political consequences. The Israeli-American relationship has always been complicated, but the US nonetheless was and remains Israel's most important ally — and while many countries do spy on their allies, getting caught doing it is generally a huge embarrassment.

In the event, the Israelis vacillated for several days on whether to admit Pollard had been working for Lakam, Israel's little-known scientific and technological intelligence agency. Eventually, prime minister Shimon Peres admitted that Pollard had been an Israeli agent, but told Ronald Reagan that the operation had been unsanctioned. Subsequently, the Peres government abolished Lakam and packed off into retirement the agency's director, Rafi Eitan (a veteran spymaster who, among other successes, had led the operation to kidnap Adolf Eichmann.)

Regarding the intelligence Pollard actually provided, it's really less a question of how damaging it was to the US than how beneficial it was to Israel. Given Pollard's extensive security clearance and position within naval intelligence, he was uniquely well-placed to supply material from across the US intelligence community. Here's just some of what Pollard supplied, according to Dan Raviv and Yossi Melman's Spies Against Armageddon:

• details of new weapons systems purchased by Arab states
• reports on Iraq's nuclear and Syria's chemical weapons programmes
• satellite photos of naval movements in the Mediterranean

But, perhaps the most striking example is this:

The most valuable pieces of purloined intelligence, in terms of enabling the Israelis to carry out a specific mission, were the aerial photographs of PLO headquarters in Tunis. There were also reports on the air defense systems of the North African states on the way to Tunisia, including Colonel Qaddafi's Libya. Israel's air force bombed the PLO complex on October 1, 1985, in the most distant Israeli bombing raid ever at that time. It flattened much of Yasser Arafat's post-Lebanon base, and Pollard took pleasure in knowing that he had helped make it happen.

However you look at it, that's a rich seam of technical intelligence — especially given Israel didn't develop their own satellite reconnaissance capabilities until 1988. Admittedly, Pollard was only one small part of the highly active Israeli foreign intelligence network, but he was a hugely valuable asset.