Reddit mentions of Iwata W-101 Spray Gun W/Pc-5 Cup

I kind of feel like the guy is talking idealism rather than realism. Realistically, of all the reasons to use two airbrushes (detail/large area), durability of the needle is really just not a concern with a quality brush. Yes, the needle will erode over time. No, it won't erode so fast that needle durability should be part of the decision making process. I won't say this guy was trying to lead you to a bigger sale (although I kind of think he is), but I will say that if you're going to pick one brush for detail and another for area work, get a proper gun for it. This would be an example of a gun used specifically for area work:

https://www.amazon.ca/Iwata-W-101-Spray-Gun-Pc-5/dp/B00OA9OUV2/ref=sr_1_11?ie=UTF8&qid=1494702486&sr=8-11&keywords=iwata+airbrush+gun

If you know that you'll be making some of the really large aircraft or ships, maybe a second gun for area work is a good choice but again it's based on the convenience factor, not the durability factor. You might not want to base coat a 3 foot long aircraft carrier with a brush that barely manages a 0.5" diameter pattern.

If it were me making the decision, I would pick a good all-around brush and see how it performs and if I find that the large area work is a bit tedious, then I'd look at a second option. Not only would you then be deciding based on your experience (rather than the questionable advice of a shopkeeper) you'll also have a better idea of what that second airbrush should look like. Do you want just a bit more coverage (second needle or second brush with larger needle) or do you want to step up to something like what I linked that can really lay down the paint in a hurry?

Either way, I'd say you don't have to decide right now. It's hard to be unhappy with a quality all-purpose airbrush.