Very interesting topic. For me, I like .8 and have been content with that. that said I haven't tried heavier but I do like .6 for wallets and smaller items. I think most people will use the .8 and heavier as it seems to be the trend to use thicker threads but I still think the .6 has a place (albiet smaller) for those that like the finer stitching.
Anything smaller than .6 is I think more of a "specialty" for those that do more luxury items requiring much finer stitching.
Anywho, I think there already is a good range of sizes and it will only be better if the .6 is added. And I'd love it if there was a US manufacturer of a synthetic bonded thread. Couldn't say I wouldn't switch over.