That answer's not exactly true, there's a list available at the hospital, but it may take a while for the list to be available to doctors and nurses. If you've visited a hospital before, they have a file on you. That file doesn't sit in the ER waiting for your next visit. It sits in a medical records depository somewhere deep inside (or next to) the hospital waiting quietly for someone to come and play with it.
I'll often respond to victims who call 911 and I'm told I can't have their medications because "the hospital already knows what I take."
While it's true that the hospital has a list of their medications, it's in a file the doctors and nurses don't see for up to an hour. More importantly for the short term, the paramedic doesn't have the list. If emergency treatment is needed in the back of an ambulance and you're taking something that interacts with a medication the paramedic is going to give you, the reaction could be deadly.
Not only that, but medications change. Even for hospitals that use computerized charts in the ER capable of providing your list of medications quickly, that list is only as good as the day it was made. We don't need to know what you were taking; we need to know what you are taking.
Some patients protest because they believe the doctor and the hospital coordinate. They do, but the wheels of healthcare turn slowly sometimes. Doctors will never tell area hospitals to update lists on every patient in their care whenever prescriptions change; the ER doctor will have to ask. Ever try getting in contact with your doctor after hours? The hospital staff has only marginally better luck.
The best bet is to have a list of medications ready to send with paramedics. Include any allergies to medications or food and put your name on the list. It's also not a bad idea to add your birthday and keep a photocopy of the list in your wallet or purse for emergencies. If you can, include a list of your medical problems, especially those that can lead to confusion or unconsciousness like diabetes and seizures.
Fancy electronic doodads aren't all that helpful to emergency responders or the hospital. Stay low-tech and everybody who needs to will be able to access your information -- every time they need it.