Your view of max_n_value/min_n_value is the same as mine but I don't see how to implement them with the generic storage either. The table has a primary key of object_id/attribute_id which I think means 1 per object. Also the get/set_attribute interface only supports 1 per object.
It seems the most used part of the data model is permissions. The meta data is rarely used. I just wonder if there would be more interest if there were more support. To me hooking the meta data to the form templating could be very usefull. The problem is I think they are some of the least used and least understood pieces of the toolkit.