Currently I have a built in Onan 5500lp genset that I use just as the OP mentioned. My prior bumper pull had a 4300 watt inverter genset in a diamond plate enclosure mounted on the rear. The portable was fine and got the job done.
However, the built in is all about convenience. Just push a button and I have instant power. No fuel issue, no hauling, no plugging in , no loading, no unloading, no security issues. No planning required. Just push button convenience.
Because it convenient its easy to use. Stop for lunch on a hot day, A.C. and microwave are available just push the button.
Decide you want to stop for lunch at a restaurant on a hot day, dog stays in RV with A.C running ...just push the button. It's that easy.
Walmart docking on a hot summer night is easy too.
Because it's convenient the built in creates lots of options and flexibility.
Portables are just as capable and much cheaper to buy, although I can run both A.C.'s with my Onan can't do both with a portable.
Portables just require more work to use them. More work maybe simply plugging in the power cord. But ultimately the portable needs to be loaded up and transported in some fashion.
Often campers leave their portable in the truck bed and run the genset from the truck which is fine until the truck has to leave for some reason and the electric goes with it! Sure you can unload genset but that just leads to more inconvenience.
I have experienced both and prefer the built in for convenience.