I would say that you do lose some skill set when you have not done it often/lately. My greatest advice to all medical students is train at an inner city/trauma center. Like Riding a bike. I trained at one of the biggest trauma center and was able to do and see everything. I will always be comfortable dealing with sick patients as I have seen the sickest of sick.
I have 4th yr med students who shadow me that goes to a community based hospital. Some have never put in a central line or intubate a pt. So go to a place that will allow you to do everything. Its a more difficult 3 yrs but it is invaluable
While I trained at an inner city trauma center, I have observed something in hiring folks for our suburban ED that people who train in an inner city trauma center DON'T get. That's the ability to relate to "normal" suburban folks in an appropriate manner. Their customer service skills and ability to relate to consultants and EMS are often absolutely terrible. They think EM is all about seeing 4 patients an hour, discharging anybody who isn't going to die in the next 24 hours as soon as possible, taking care of trauma, and resuscitating folks circling the drain. The "softer" skills required for a happy career with plenty of longevity are taught much less often in an institution where you are continually bombarbed by "an inner city population" who doesn't take care of themselves.
Now, most EM docs are smart enough that they can pick many of these up in their first 6 months in a suburban ED, but they certainly don't come out of residency with them. Now, what are some of these skills and how important they are can be debated, but I assure you they are important to the people hiring you.
Things like:
Knowing when to get an MRI in a hospital that encourages you to order as many as you like
Knowing how to say no to a drug-seeker while making that drug-seeker and the facilitator with them think you care about their health
Documenting in a manner such that you are rarely downcoded
Documenting in a manner that will keep you out of the court room
Defusing upset patients, especially when they're upset about stupid stuff
Selling a patient to a consultant such that they don't "block" admissions
Working well with a set of consultants who will generally give orders by phone, NOT come in to the ED to see the patient- everything that needs to be done tonight will need to be done by you
Knowing when and how to transfer a patient despite the fact that your hospital is supposed to be able to do what the patient needs, but you wouldn't let them do it to your mother or child
Defending your contract from a CMG
Encouraging EMS to bring you MORE patients, not less (why nurses and recent EM grads do this is beyond me)
Figuring out how to keep administrators happy so you can keep your job
Getting a consultant to do the right thing while preserving a long-term relationship
Dealing with toxic personalities in consultants or partners or administrators
Finding help for uninsured patients in your community
Getting nurses to respect you (particularly tricky for female docs straight out of residency)
Doing appropriate work-ups in patients with psychosomatic issues- when 3/4 of your ED is full of patients having them, while convincing said patients to not only consider anxiety as a possible cause of the symptoms, but also to seek care for it.
Knowing which patients go to medicine, which go to surgery, which go to cardiology etc (different in every institution)
Knowing when to do your own US, and when to let the radiology department do it
Understanding the concept of a "Medical Screen Out" and when and how to use it
How to effectively use your state's CSD
How to maintain skills on procedures you do rarely
How to restrain patients and their family members when your only security guard weighs a buck twenty and you only have three nurses and a clerk, all female.
I could go on but you get the picture. An inner city trauma center is different from most EM jobs, and so can't provide complete preparation for most EM jobs.
That said, some things you also must learn are very difficult to learn outside that setting.