My understanding from AMK's site is that the Emergency Bivvy (the little one) is meant to be more or less for use on a single excursion just in terms of the durability of the materials. The Thermolite II Bivvy is meant for multiple uses.

I haven't used either of mine, but from removing one of each from their packaging and then re-rolling and stowing, I can see from differences in the materials why the one is "disposible" and the other is meant for reuse.