Totally depends on the injury. Commercial jets are pressurized to the equivalent of 8000ft or less, a lower altitude than say, Vail ski resort, so generally, pressure shouldn't be a huge issue unless someone has a sinus/ear infection or general airsickness (the two biggest medical problems unique to flying). You could hit a pocket of turbulent air and end up with 5 traumatic injuries. Someone could have a heart attack and need aspirin and calming down, speaking of which, someone could just be having a panic attack and need a talkdown. There are as many possible injuries in a plane as there are any other one place. An EMT could help with any one of these things, and although its possible the flight attendants would be perfectly capable of handling these situations, in some cases they might now be.
O.k., I'll bite.................
1. So you have 5 traumatic injuries from turbulence, that is actually a quite rare occurance. Every commercial flight plan includes an up to date check of current weather and pilot reports. In other words, turbulence is very rarely an acute unknown issue. If clear air turbulence is reported or observed greater than light-moderate, the seat belt sign is going to go on. So that aside, what you going to be able to contribute that will improve the injured persons outcome that could not be provided by any other passenger on the plane or the flight crew?
2. The heart attack scenerio. Are you carrying the Aspirin? If not, how are you going to administer it? Are you appropriately credentialed to do so?
3. Same with the panic attack, others can offer the same level of assistance.
4. You are correct, pressure shouldn't be an issue, but in an emergency, it is a huge issue! The flight crew is well versed in the understanding of the physiology associated with flight. Much more so than an EMT-B and in many cases, an EMT-P. Case in point, i am interested in hearing your explanation as to where you came up with the 8,000ft. figure?
Yes, an EMT can be as assisting as anyone else, but the key here is dependent on if your help will be requested, accepted, or even allowed. That decision is made by the Captain and the decision he makes is the final word and law on that flight.
Your enthusiasm is admirable, just understand your limitations and look at the whole picture before flying down the aisle.