Metal modification is SO unaesthetic
"Oh look at that freak, he's installed a steel hand! Ewww! His grip is as cold as a grave!"
After a person would desire making himself a mod, especially if not directly required by his physical condition, he might become unpopular due to xenophobia that might be prevalent in high society. Like, he's turned alien all of a sudden, and the main trend was to get away from aliens. Also some mods might be uncomfortable for others just to see, like a Terminator's eye that allows night vision, IR and probably some target tracking, on a body guard, leading to decreased wage of that person, even if he's doing his job better than before. High-level society is so brittle, you know.
A middle class that's suffered a traumatic amputation of an arm and does not have enough money stored in wallet to immediately buy a replacement (anyway normally it'll take a while to get one, medical treatment, tailored production, post-implantation adjustments, etc etc) would have to get the extra money from somewhere. Banks are normally happy to provide people with credits, yet they demand payment from them for years to come, and that person's budget could well turn into a downward spiral. Especially if the new arm would require expensive maintenance every so often.
Money-based reasons could be many, starting from "license issues" to "pay as you go" somehow wired to movements made by the attachment, even our current car manufacturers have a lot of practice to drain people of cash, let alone something entirely information-based like recent Unity. Just invent some simple but expensive money sinks for the implant user, there you go, effective declassification.
Temporary mods inflicted brain alteration
An example: there is a drug that increases your metabolism in the way that you have become stronger, more agile and more perceptive. A person in question used that several times (as this mod was temporary) and accidentally discovered that his brain had adapted to the drug (and not just the new abilities) and now pains hardly when there's no drug (aka addiction, but biological rather than psychological). That, and the sense of being superior while on the drug turned the person wild, inducing psychical disorders as a consequence. He goes drunk or drugged by something else and falls off the society. However, he can still get a position in the lower class, as a brute force for example, working for food-and-drug just to not get fired into oblivion.
And a point missing in the above answers:
Mind-machine interface required for metal implants turns people less socializable
I think, and my thoughts cross the barrier into the synapses of the machine, just as the good doctor intended. But what I cannot shake, and what hints at things to come, is that thoughts cross back. In my dreams, the sensibility of the machine invades the periphery of my consciousness: dark, rigid, cold, alien. Evolution is at work here, but just what is evolving remains to be seen.
Commissioner Pravin Lal, "Man and Machine"
Considering that high society requires some etiquette to be in, and that modified humans might have troubles tolerating it, they might as well get shunned off the beau-monde downwards, potentially even not being able to withstand social pressure from ever accompanying humans. Some people might even get psychotic from even a successful attempt of installing a mod, whatever that was, but since most mods actually make the "remaining human" more durable, or at least designed so, the affected human would likely stick at a job of some lower class than where he was initially. Probably in an army, as MMI as is is mostly used as a war-oriented alteration, and a person with an installed MMI would cope better than those that don't have one.
On the other hand, biological-based mods should perfectly do
Imagine a golden youth who's got his liver damaged by excess alcohol and drugs. His dad managed to have him undergo liver replacement with a modified biological organ, tailored to the kid so he's got some poison resistance additionally to general well-being. Since the organ is biological, and whatever scars from the surgery might get masked (we're speaking rich people after all!), the kid looked normal after undergoing a rehab, with a consequence of utter aversion to alcohol (whether the liver was the reason, the dad preferred to not disclose). On top of that, he's got his wits up, dropped the company that otherwise was leading him into a pleasure pit, started studying and eventually succeeds his father's position in a large company, together with riches. But, he might decide to not disclose the data on his past treatment to the public, so he's a "cyborg" of sorts but still in high society that thinks he's just a kid recovered from bad habits.
Or another case, a person lost his arm, and instead of opting in for a metal replacement, he went the more advanced but more expensive route of growing a new arm in a lab with eventual implantation instead of his old arm's remnants. I would say that person would get positive score among its peers, both for having a "real" arm and for suffering a lot while he might just get a steel arm quickly. The same would work with internal organs, eyes, ears and other visible parts.