The most potentially damaging aspect of this latest Obama letter is that U.S. allies in the Middle East weren’t informed of its existence.
This is actually the least worrisome part of a not-very-worrying story. First, the U.S. isn’t obliged to make its every communication with the Iranian government known to its regional clients. It would probably defeat the purpose of some of these communications if they were shared with a large number of other governments, especially when many of those governments are adamantly against reaching a deal with Iran. Now it may be that sending this letter serves no purpose, or it could possibly even undermine the successful conclusion of the negotiations, but then that shouldn’t bother the regional clients that oppose the deal.
The only reason why these clients would be alarmed by this letter is if it significantly improves the chances of reaching a deal that satisfies the U.S., Iran, and the rest of the P5+1, but that is also why its contents should have been kept from them. The U.S. shouldn’t have to keep reassuring its perpetually whiny clients about everything it is doing in the region, and it shouldn’t have to clear its diplomatic gestures to other governments with its dependents. If U.S. clients in the region were committed to supporting diplomacy with Iran and were invested in a successful deal, it might be a different story, but they aren’t and we all know that they aren’t.