Simply put: positive (transparency) density achieves a crisp, clean screen image and also negates problems with washing out the image from an exposed screen. Most inkjet film has a specially developed coating that is designed to work with the standard Epson inks or for much higher density and clarity when using Blackmax high density black ink. The ink dries in seconds and delivers superior performance for screen printing and plate burning applications for pad printers. 

With a positive that is not opaque enough you’re not blocking 100 percent of the light. Instead you’re cutting down the amount of light by 30-50 percent. This in turn will make the result look like your image has been burned onto the emulsion when in fact it will not wash. You are over exposed at this point. If you use a pressure washer to remove the half exposed emulsion you weaken the ability of your design to withstand a good production run, or it may not wash out at all.

You will sacrifice clarity, longevity and stencil strength; not to mention the excessive pinholes and premature stencil breakdown that is likely to occur. Reclamation problems will also be a factor as you will likely lock in the emulsion from improper processing.