As Max has stated on the forum previously - the implementation so far is a first step towards a more capable system for materials.
Previously materials only had density and hatch - so there are no other properties to show, though the spaces are there ready for them.
Stefan is correct, it's possible to edit the resulting XML file if you wish. Possible, but not encouraged.
I presume that in a future version, the Alibre Library materials will either have the property values filled, or the user will be allowed to edit it (or both) - perhaps Max can comment on the plan.
Until we know what is planned, there is the potential that effort put in populating properties in the Alibre Library might just get overwritten when the next version is released. So I'd urge caution before rushing into doing this. By all means edit / populate your own custom MTL set.
Previous functionality for materials (not that there was much) has been preserved whilst introducing a format that is capable of much more in the future; so I'm a bit surprised to see such negative reactions.