What is this piece?

My guess is that it an attempt to give some light rain protection with the breathability of a windshirt. With a pack on only the front would really have any exposure. I think it would be great for drizzly PNW rain. At 5.7 oz, I'd like to try one.

I agree, I've thought for a long time that something like this would be useful. Think, if you are hiking hard in temperatures between say 10-20C, and its raining, and you have a pack on, which covers most of your back (and just gets completely sweaty with any jacket) plus shoulder straps and waist belt on the front, you just sweat it out. Much better to just let yourself get a bit wetter from rain in some spots but have much greater breathability/drying ability.

With my wind shirt (Tachyon pullover) I probably wouldn't miss the breathability losses of a full waterproof hood or shoulders. If I'm too sweaty under there chances are I'm too warm and I unzip and remove the hood anyway to vent. Same in the rain, actually.

Oftentimes the greatest velocity vector of rain is downwards. The most perpendicular parts of a jacket to rain, and hence the parts experiencing the greatest hydrostatic head, and the first to wet out on a wind shell, are the hood and shoulders. Rain has a much better chance of just glancing/beading off the front/back panels and arms.

I would like to see a lighter version of this though. 5.7 is a bit heavy. Taking a normal windshell and a wide brim goretex hat would beat this possibly and provide almost the same benefits.

I hope Montbell take notice (actually don't, I just bought my Tachyon a couple of months ago, I don't want a better version to supersede it too soon).