Another issue with cutting bevels on a miter saw is when the boards are cupped. It cuts through the cupped board but doesn't follow the line. When you go to install the board you either have to leave the cupping and try not to flatten it out while nailing it (and the cupping will likely change with humidity), or flatten the cupping out which pulls the straight cut into a curve. It's much more problematic building decks with wet pressure treated wood as the sopping wet wood will cup worse than something kiln dried and flat to begin with. I'm glad my deck building days are behind me.
I had better luck going freehand with a circular saw on badly cupped boards. For stop cuts I couldn't make with the one way bevel of my saws I would use my cheap Harbor Freight backsaw. I have horribly abused that thing and it still cuts really well. I've also pinned the guard back on my circular saw and ran it backwards, but that is outrageously dangerous. The backsaw cuts fast
enough that it is a great tool for the job if you only need it a few times a day. They make a bunch of different pull type saws which use thinner blades that are better suited to the task than push type hand saws.
If you are doing trim all the time then hopefully you aren't working with terribly cupped stock and the double bevel action could save time. It probably isn't worth paying extra for it as a weekend warrior, but if they were the same price it could be a handy feature on occasion.