Under many circumstances I would agree with your take on this. However, there are certain jobs that should have a basic standard of morality and particular situations where personal behavior have direct implications on job performance. The multitude of problems with the USSS appear to span both of these categories.
Law enforcement officers, and particularly those with 1811 designations, carry tremendous power. With great power comes even greater responsibility. While people like to think that someone can be morally bankrupt in their personal life and a pillar of society at work, I've never seen someone flip that switch successfully. That is to say, people who are jackasses off the job tend enevitably slip up on the job. Foreign intelligence agencies love to use personal indiscretions as leverage. That is why the BI for 1811 jobs is a virtual colonoscopy into the public and private dealings of the applicant.
In the particular case of the Columbian hookers, we see this phenomenon displayed on a very personal level. One of the agents involved in this scandle was a married father. That didn't stop him from carrying on multiple affairs with women across the US and abroad. In Columbia, he got shitfaced with hooker that he brought back to his hotel room. After having sex with her, he proceeded to pass out with the hooker in the room having full access to his credentials, weapons (keep in mind that he was on the Counter Assault Team), and undisclosed sensitive items. This is a perfect example personal behavior catching-up with an asshole until it eventually bled over to his work performance. This could have been prevented had his supervisor hauled his ass before the OPR when they first heard about his harem across the globe. Instead, they knew about this behavior and even ran cover to hide it from his family living in DC.