My two cents....the flush trim bit is designed to be used to ride the surface of either a straight edge or more often a template. It’s mostly intended to be used for making copies of something, but can be used for straight edge cutting as well.
While it in principle is no different than a straight bit here’s why I think you should avoid using it as one. The value of the flush trim bit is in the bearing. It’s why most experienced wood workers have given me the guidance of using it to cut minimal amounts. You want to protect the bearing from gumming up and if used to do heavy amounts of cutting it will increase the likelihood of that happening. In addition the pattern bits are all 1/2” in diameter, whereas most straight bits are 3/8” (there are of course various other sizes). Using the larger diameter means more material being cut away which means more resistance, heat, etc that can contribute to the wear and tear on the bit. Using a 3/8” diameter means less material, less resistance and less heat, and no concern about protecting a bearing.
Having said all this, the bearings are replaceable (something I didn’t learn until more recently). I have used the top bearing bit to cut A LOT of straight cuts. I’ve since relied heavily on the track saw, it’s easier to setup and doesn’t remove as much material when cutting.
So yeah you can use it, but I’d start investing in things for their intended use, hence the straight bit where you can, and the pattern bit when you need to. Hope that helps.