Why Layer Effects haven't any preview windows ?

Hello every one
First thanks from JonStipe for his great layer effects plugin for gimp.
My question is something like a feedback :)
Why layer Effects plugin haven't any preview windows?
When I use this plugin I'm like a blind man and my final result is really fortuitous and I play with numbers to find my custom result and that in 80% isn't my custom result.
Please add a preview window to this plugin.
thanks again.
Regards Samad