Very interesting stuff. I think it's more than a case of personal preference, there is also appreciable skill in both methods.
I wouldn't say you were 'Missing out' as such, because, as others have pointed out, there have been some truly magnificent models on this forum all painted with a paint brush.
But the difference is that those people are very skilled with the hairy stick, and that brings me to my main point.
I am NOT skilled with a paintbrush, I am, frankly, useless with a hairy stick. When I brush paint it looks like it's been put on with a trowel. Consequently I moved over to an airbrush simply to get better results.
And like others have said, I have never looked back. But the important point is that I am not advocating that an airbrush is the only way to go, I am saying I use one because I am rubbish at brush painting! If one is very good at brush painting then it could be argued that an airbrush is a superfluous luxury. So the question is; Are you happy with your results with the hairy stick? I wasn't, so I got an airbrush, but if I had been getting results I was satisfied with then I may not have bought one. See what I mean?
Personally, I have never looked back, I airbrush absolutely everything (except the very smallest parts, such as pilot figures). I find airbrushing much, much quicker, smoother, better finish and far more economical with the paints.
For example, It takes just seconds to shoot the interior of fuselage halves with an airbrush, much faster than poking into every crevice with a hairy stick, and a blast of thinner through it afterwards and it's ready to go again for the next colour.
Once every couple or three models I give it a 'Proper' clean up, but even that doesn't take me too long.
As for cost and investment, I went very cheap with compressor and airbrush, but I am quite happy with them both. Plus you save a lot of money on paints, you use far less with a thinned down amount in an airbrush than brush painting a similarly sized area.
So, for me, I embraced an airbrush simply because I was rubbish with a paint brush!
But then, at the end of the day, it's the finished model that counts, not how many clever tools and techniques are used along the way. If one is happy with the resulting model then it doesn't matter what was used to achieve it, it more depends on the skills in actually using those tools (whether paint brush or airbrush) and being satisfied with the results.