Hi Randy,
I could do what I need using other methods, but there are a couple of reasons for going down this route, one is that I use the code is a few places and this is exactly what EPICOR functions are designed to do, to provide a central / common code base wherever possible. The only way to call a function in an EPICOR form customisation is using REST there are no business objects available to call here.
Also, we have our eye on the eventual move to Kinetic forms and are expecting that using REST within the Kinetic studio may be the way to go.
All in all it is a bit experimental, but we love a learning curve!