I still have the AF200T and Soligar 14s slave, that I owned when I got my Program Plus way back when in the 90s. I do use the built in flash from time to time. But a GPS would be better for me.
I find it interesting that people now have to have stuff that used to be optional. I guess for those of us for whom flash was always an ad on, its sounds kind of silly. For those of us who grew up with on board flash, they expect it. But for folks who lived in the 80's and 90's, only the cheaper cameras had on board flash. So there's bit of a great divide here.
The advantages of a separate flash are pretty much endless. Separate batteries, so your camera battery isn't drained by the flash, bounce flash, much more powerful flash. My AF 200T recycles much faster than my on board flash does. Even my $19 Soligar 14s does, and it's powered by 2 double As. And I can carry the Soligar in a pocket of my camera bag or s shirt pocket. There is a power adjustment on the back of the flash. I see no need to adjust it in camera. But maybe that's because that's what I'm used to.
From my perspective, I find it odd that some one would pass on a camera that's $1000 cheaper than it's closest real competition, over an issue that most of us learned to deal with years ago, and have dealt with stress free most of our lives.
So, ya, I agree with the salesman. If you want to consider yourself to be a pro, get used to dealing with off camera flash, for about a thousand reasons.
Go ahead, laugh at me, while I'm laughing at you. We can laugh together, just at different things.