All I can say, is when the Army transitioned to the "new" Branching Model where you pick, but the Branch has the final say as to "Most Preferred", "Preferred", and "Least Preferred", it was supposedly done for a seemingly good reason...RETENTION! The services have all had issues with a lot of people getting out (both officer and enlisted) at that point that they were trained and important cogs in the scheme of things. Generally at the O2-3 and E4-5 levels members trained and qualified were leaving because either they made the wrong choice in specialty, or didn't anticipate the actual job requirements and were unhappy in their choices. When you mention BRADSO (which I think is what you are calling manipulation), it tells the Branch or Specialty that you as a service member are really sure about your choice and are willing to commit to more service time. Both Officers and Enlisted have mechanisms for this, ADSO for schools, or promotions.
But to claim "manipulation" is a stretch. For years nepotism has existed, "Godfathers" looking out for their friends children and their careers. If that is not manipulation, then extending your ADSO for additional schooling, a promotion, or a particular field is not manipulation. JMHO, but EVERYONE at least at WP has the opportunity to BRADSO, and if not needed you are not charged, so everyone has the same opportunity. If you don't want to serve more time to get what you want, then that would be your choice, but don't say the "old" system was better until the new system has a few years to see if retention numbers are better.
Again JMHO, but no system is perfect, nor will any system make everyone happy.
One of the factors leading to the change was retention and I understand the reasoning put forth. However, I have never seen any data to support the hypothesis that branch assignment by Order of Merit was a cause of dissatisfaction and lower retention. Insufficient branch education may have been a driver, but that is more readily resolved through increased branch familiarization and additional opportunity for branch transfers rather than a change in how selections are made.
When I refer to manipulation it is not in reference to BRADSO, which I think is a bad idea for different reasons. By manipulation I mean the gamesmanship which occurs in the process by both Cadets and Branch reps, and influence exerted by outside actors. A couple of actual examples and by no means outliers:
1. Cadet A, top 20% of class and good physical/military grades, has consistently ranked Infantry in the top three and had always been ranked "Most Preferred" by the Infantry branch rep. Likewise ADA has been near the bottom preference and Cadet A had always been ranked in the middle, "Preferred." At some point later in the process Cadet A's preferences change. Infantry drops substantially and ADA moves into top five. Cadet A then receives a "Most Preferred" from ADA and an invite to lunch with the Branch Commandant, who flies in from D.C. to help recruit the branch's "top candidates". Simultaneously, Infantry drops Cadet A from "Most Preferred" to "Least Preferred".
Learning Point: A Cadet's preferences have a significant impact on the rating by the branch. You can "schmooze" your way to a higher rating, something that can not be done by Order of Merit. Branch reps have a vested interest in having as many "Most Preferred" and "Preferred" in the actual selections as possible and can adjust ratings as needed, irrespective of the quality/fit of the Cadet.
2. While doing an internship at the Pentagon, three Cadets meet the Chief of Staff of the Army. At the end of the conversation, the Cadets are told to give their names to his Aide de Camp so he can make sure they get their first choice branches. So much for the rigorous rating process to get the best "fit". Even if the CSA did not follow through or if the Cadets misunderstood/exaggerated the interaction, the perception is enough to doubt the system and call into question the myriad of other opportunities for interference in the process.
Learning Point: In an opaque and complex system, a chance occurrence having nothing to do with the process can affect the outcome. In the prior system, that fortuitous meeting would not have changed those Cadets' place on the Order of Merit.
Food for Thought:
1. If a Cadet's preference is so significant to the branch rating, why bother with such a Rube Goldberg process.
2. If a branch rating determines who is most and least fit for a branch, why are Cadets forced into branches for which they are not a fit.
When there is no ideal solution it is better to choose a simple, fair, and transparent system. Determining branch selection by lottery would optimize those three criteria, but could have the unintended consequence of reducing incentive to excel. Thus, we are left with merit - a dirty word in many places today, but once the hallmark of West Point.