Warning letters are not PRIA reportable if I understand them, so it's between the pilot and the FAA and by definition and it is a completely confidential item not to be disclosed by anyone even when asked if about it if they do not wish to (ie. on job apps). As JNB said, the FAA uses a warning letter to let a pilot know there is a penalty coming soon if they do the same kind of thing again. It is not meant to slow or stop a career, but if the pilot is doing things he shouldn't then what else can the FAA do to correct them? And how could the letter have any adverse career effect if the pilot does not disclose it? It's a private document and does not have to be disclosed to anyone other than the pilot who gets it. There is no official or unofficial (ie. ethical) need to disclose it even on a job application. Consulting a lawyer is a waste of money over minor, non-recurring violations when the pilot is indeed guilty of doing the deed, they are not a repeat offender and they probably won't do it again any time soon. The only way a warning letter might be viewed as out of line in my opinion, is the occasional instance where the FAA actually has the wrong man. In such cases you should be able to get the FAA to drop it anyway during the first phone conversation with the investigator.

What I think USMC was saying (or I'll say) is, that if you did the crime then do your time, take your lumps and grow up. Nobody is perfect and anybody arguing otherwise is a fool. If you lose a job because you disclosed a private warning letter voluntarily, then that's your fault. In my view, hiring a lawyer to get you out of some minor infraction is 1) disingenuous, 2) undermines the basic penalty system, 3) costs money, 4) wastes everybody's time and 5) does not make anyone a better pilot.