You are confusing this "feature" (actually a hack) with a new feature which exists as a dormant one in v1.23 viewers and that LL will probably implement in the future, at least if they can find a way to make it compatible with existing contents: this dormant feature is the possibility offered to use the alpha channel of skins to allow to hide body parts (including the entire body if needed). However, as it is, this feature can't be enabled without breaking many existing skins.
Not at all... This would break almost all shoes, and a lot more contents.
For info, the invisiprim feature is just a special texture (that you may as well export to OpenSim grids) applied on normal prims.