The best way to think about the Secret Service is to realize that its name is bogus. It isn't secret. Instead, it's like any other government agency intent on maximizing its influence, reach and numbers. It can always dream up new threats that it needs to address. Congress has pretty much signed off on anything the agency wants. Why not subject the Secret Service to the scrutiny that other government agencies are now receiving?
Ambinder is more sympathetic:
The Service can be arrogant when it comes to aspects of its protective mission, but has always exuded a sense of humility about itself. It does not have, like the FBI and CIA, any employee dedicated to working with Hollywood producers. It rarely gives reporters access to even its least sensitive operations. It does not leak, a rarity in Washington.
Humility and discretion clearly went missing in Cartegena, in a serious lapse for the Secret Service. But the lapse did not occur amongst those who protect the president most closely, and so the ultimate damage done to the Service will probably prove to be lighter than it seems today.
Earlier Dish on the president's trip here.