Your importance on an A-Team can be defined by experience and training.
If all you are is a Jr. 18B you are low man and the first one I will ask to break the seal on your mask to check the air quality.
Now when you've been there a few years and have both 18B, 18E, now you're an asset to the team. same goes for 18C/18B and so on.
And if you're an 18B that came from an infantry/ranger background, even better.
One SF MOS does not usually make a "critical" position. That one crit spot/MOS is held by the 18D's.