Just wanted to say thanks for all the great info you've presented us with frank, I read through this thread for the second time today and wish I had found it when I first started with planted tanks!
EDIT: Frank, any chance you could enlighten me as to what the benefits of doing small rocks covered in moss is versus coating a single piece of wood or a large rock?
Thank you for your kind words of support pmcarbrey!
As for your question, it's not about a specific advantage of small rocks versus driftwood - the only exception id say is with a large rock.
Small stones allow flexibility in where and how you place the moss around other plants and arrangements (such as on or between stones). This adds a layer of detail work to your layout.
Attaching moss to driftwood is another application of moss all together, but if you wanted to carpet moss, it's a different deal.
As for big stones, unless you have a big aquarium, this diminishes your flexibility of planting around it and I definitely recommend against it. Additionally, except for propagation, I do not even prefer tiles of moss (unless the entire carpet is moss).
For example, had I used a single flat big stone to tie moss to in the Mini M version 1, then I would not have been able to mix tenellus or hair grass with the moss to create the effect that I did.
Additionally, plants like moss and Riccia have a tendency to take on the shape of the object they are tied to - so the effect of moss tied to small stones makes natural contours in the layout and add an extra layer of depth to the overall picture. Contrarily, if tied to a tile the whole appearance takes on a flat appearance. Certainly desirable in some cases but not in others and not in most cases where I use moss.