I do agree with you about not enough comments being posted on photos of pics in GARDEN SHOWCASE, but I did notice something strange about the process "itself" of posting comments. When you look at a "garden" you can post a comment on each photo individually - AND you can also comment on the entire album as a whole if you wish. The comments on the whole album are viewable to everyone. BUT if a comment is only posted on an individual photo, it is viewable, but ONLY on that individual photo if you click on the comment button that shows there is a comment posted.
I really don't think the Garden Showcase is designed in such a way it leads to a lot of comments back and forth. After the showcase is posted,unless people go back and tag photos, they may seldom go back and view their showcase. So it might be weeks or months before they see the comments. For example if you have over 20 pictures in one of your albums, plus you may have several albums, what are the chances you are going to go back and look at all the pictures in those albums to see the comments? I think this is mostly designed for a search engine type of thing for plants if people will actually tag them, and for having a visual look at different gardens that might inspire ideas for your garden.