People who care about optimizing their builds flock to the same advancements regardless of what they must do to take them. Pick a right homeworld? Sure. Pick the best career path? Right. Switch for an alternate rank whenever possible? Absolutely? Pester the GM until he lets you take it as an elite advancement? In munchkinland, it's do or die!
No system can hold back a devoted power gamer. The only person with that kind of power is a good GM.
Also, let me tell you a story about alternate ranks in previous games:
When I started running DH, the group consisted of a Cleric, a Tech-Priest, an Assassin and a Psyker. After we got our hands on Inquisitor's Handbook, the group consisted of an Inquisitorial Legate, a Mechanicus Secutor, a Moritat Reaper and… still a Psyker, but only because he already entered the Scholar path and thus couldn't qualify for Templar Calix.
The first time I played RT, the team consisted of the Rogue Trader himself, a Void Master, an Arch-Militant and an Astropath. The next RT game we cooked up took place after Into the Storm and started at high exp level, and thus we ended up with a Genetor, a Flight Marshal, and a Rogue Trader whose main concern was qualifying for Legend of the Expanse.
The same thing happened in two different DW campaigns I've played in. As soon as Rites of Battle introduced new ranks, everyone jumped at them like Blood Ravens seeing an unattended bolt pistol.
The same thing happens with all the background packages, alternate home worlds, Distinctions and whatnot. That's because all these options introduce either direct, honest to god power creep, or at the very least "coolness creep" - even if the new stuff isn't just plain better than the corebook stuff, it's still worth taking for the fluff alone.
Now, I do appreciate many new options that all this stuff offers, but I can't help but think that by packing it the way they did, the authors stiffled my team's creativity more than they encouraged it. Why think up a cool forge world or a cool twist to your omnissian creed when you can join the well-defined and admittedly flavorful Cult of Sollex? Especially since such a background makes it only natural for you to follow the vastly superior path of Mechanicus Secutor… and so on, and so forth.
Now, I haven't seen anything like it in either Black Crusade or OW so far. People really think a lot about making their characters unique when they can't easily default to a pregenerated template.