Aggie Blue said, "...can you see specific problems that result from making loyalty a guiding principle?"
Yes, if loyalty gets caught up in whether or not to lie and possibly do an unlawful thing then problems will arise. For example say you were called as a witness and were asked under oath about a situation involving money embezzling and the CEO of the firm you worked for. An ethical decision will need to be made and hopefully loyalty to yourself and your career will be the decision you choose. The one you were so loyal to before will not be protected (hopefully) because lying in high profile situations are not worth loosing credibility, your good name and status in such a competitive business for.
Journalists and public relations practitioners need to be loyal to themselves their profession and to their audience. Loyalty ranks very high on my scale of professional values but like my example there will always be problems. Deciphering when to be loyal or when to step back to protect yourself is the tough part.