This basically comes down that you want Arena to be more like Madmapper. The history of these softwares is different though.
Madmapper was originally developed by 1024 as a tool to make their projection mapping jobs simpler. You can really see in the interface and workflow that they did this a lot, and the tools are really custom built for warping to strange shapes with a projector. It's an awesome piece of work.
The advanced output Arena was originally developed as a special edition of Avenue 3 for EyeSupply to make their LED pixel mapping jobs simpler. This is why the focus is more on working with multiple outputs, rectangular shapes and pixel accurate positioning. If you've done any pixel maps, you'll know what I'm talking about.
As Arena progressed, the advanced output evolved to a tool for projection mapping, similar to how MadMapper can be used for LED pixel mapping as well. You can really tell the difference in the original approach though, and there's no denying that in specific cases, one tool will perform better than the other. We're enjoying the fact that developments on each side is pushing the other forward, and in the end you as a user reap the benefits of this.
Somewhere down the line we'll probably think of some clever ways to allow for non-rectangular, non-warped slices as well. For the time being though, I hope this explains where we're coming from.