Initially, I implemented this to be interoperable with #Honk, so this would break Honk compatibility, as #microblogpub user would not see locations anymore from it, and followers using Honk would see the location twice.
Personally, I am not a true believer in linked data, far from it, but I do think it's better for easily interpreted (meta)data to be encoded usefully, as opposed to dumping everything in the content field. Embedded hashtag and mention links never actually go where I'd like for example.
There are many devices for which an OSM link is not the ideal handler. Some duplication of information isn't pretty, but it works.
More philosophically, limiting ourselves to the most minimal feature set is no fun. Users of platforms that are missing out can always file feature requests. :)
Will probably get more buyin from other projects if we stick to the field that's defined in AS.