If for no other reason, because MONA can “clean up” a 12 lead ECG, making it non-diagnostic by arrival at the hospital!
*** Update ***
Here’s another case (from several years ago) that helps illustrate the point.
This was from a chest pain patient who was transported to my jurisdiction from a neighboring EMS system by boat.
In other words, it was an EMS transfer. We met them at the dock and took the patient to the hospital.
Here is the ECG that was handed to me by the transferring paramedic (who recognized it was showing acute inferior STEMI).
I’m pretty sure it was recorded by a LP11.
Here is the 12 lead ECG I captured en route to the hospital (using the exact same electrodes).
I’d love to tell you that the (first) prehospital 12 lead ECG saved the day. It didn’t. I personally showed it to the ED physician, but he was dismissive. No urgent action was taken.
I have no idea what ended up happening to the patient.
* Note: You can still identify acute inferior STEMI in the second ECG by the Q waves and non-concave ST segments in leads III and aVF, along with the downsloping ST segment in lead aVL. However, it’s the first prehospital 12 lead ECG that really tells the tale!