First to the weenies who think anything larger than a 22ga is torture - you know those needles the Red Cross and other blood donation organizations use to draw out a unit of blood from donors? They're 15ga. THAT is a large bore needle, as are 14's and 16's. An 18 is not. There are commercially available 10 and 12ga IV catheters available.
Now, from my anesthesia/surgery standpoint. If you have a trauma patient in the field, I will sing praises to your name if the patient comes to the OR with a 14-16ga IV in place. Conversely, I will probably curse you endlessly if they roll in with a 22 in the ACF. You may see the patient before they get into shock and when you can actually still find a vein. If it's a trauma patient, and you see a big vein, PLEASE place something larger than a 20. The further down the shock road they go, the harder it is to get a peripheral IV and the higher the chances that we're going to have to get central access. I MIGHT not have to put in a central line if I have a peripheral IV that runs well. Central lines are not an innocuous thing to do and have lots of nasty complications.
Those of you who think a 14-16 is "out of style" simply don't know what you're talking about, because in the proper situation, a really big honking IV (as we call them in the South) is a blessing. A 20 on a trauma patient is simply too small for any significant volume resuscitation or for blood. An 18 is better, but a 14-16 is da bomb. Hey, if you don't want to flood the patient with fluids, fine - just turn down your flowrate. But for those of us that deal with the patient shortly after you bring them in, my fluids/blood/FFP/colloids/multiple drips will go in much better with a larger IV rather than a smaller.
Oh - and as far as injecting IV contrast dye - using a larger bore IV in a larger vein such as the antecubital is desirable for several reasons, the main one being less chance of infiltration/extravasation. IV contrast is thicker than IV fluid and harder to inject. The tendency is to push it in - and of course when it's hard to push it in, most people just push harder, which leads to extravasation/infiltration, which is not a good thing with IV contrast. Having a larger catheter in a larger free-flowing vein makes for happy patients and happy radiology techs. But if all they have is a 22 in the hand, it'll do. The tech will just complain more.