Remember, a Rosette isn't just a paperweight. It also has a collection of Inquisitorial identification and override codes and ciphers, etc. Any Arbites Precint Fortress - any Imperial Adeptus Terra facility with functioning tech and lay-technicians, really, and probably any Imperial Ship - would be able to verify that a Rosette is really a Rosette and the bearer is really the Inquisitorial Representative they say they are.
However ... part of the process is the Inquisition's reputation. They Do Not Approve when someone claims to be an Inquisitor who isn't the Inquisitor they say they are. For that matter, in DH's Daemon Hunter sourcebook, there's a fluff bit wherein an Ordo Sicarius(I think) Inquisitor who pretended to be an Ordo Malleus Inquisitor got executed for impersonating an Inquisitor.
The vast majority of people wouldn't even dare to consider impersonating an Inquisitor.
Also, if you're going to impersonate an Inquisitor, and get away with it for any amount of time, you're going to need to be very careful with how and what you do with your false authority. And, of course, if you ever get exposed, the Inquisition will promptly place a very large price on your head, dispatch Imperial Assassins, Imperial Navy assets, their own ships, and quite possibly raise up a Rogue Trader Warrant with the condition of bringing you and ship in to be made an example of and seize all your assets. All depending on what you've done and how well resourced you are.
Even Chaos doesn't impersonate Inquisitors much - Chaos may draw Inquisitors into Radicalism and Heresy, but that's a fallen Inquisitor (whose radicalism and heresies may or may not have been exposed yet), not a falsified Inquisitor. There is a difference and it does matter.