I can't answer all your questions, but I will point out that the advantage of structured data is the chief benefit of a "database" rather than a "forum."
The forum (and comments to a plant are a sort of forum) are free form, readable, but not really searchable because of their inherent lack of structure.
Structuring the information in the way we're talking about gives us a lot of power in presenting that information to the users of the database, and also lets us enhance the database even more with powerful search options.
Looking at the database as a whole, we could do without all the data details and just make it all comments. Instead of checking off "Annual" and "Full Sun" and "Edible Fruit" you could just put that all in a comment. But then you can't go and do a search to see all plants that are annuals, needing full sun and producing edible fruit.
