I would find it extremely helpful to have an offline version also.. perhaps a 'lite" version where the article tab is available, and perhaps the spun article tab also. Having a downloadable database of synonyms, or no synonyms available during offline times, would be completely acceptable to me. I would love to have a version I can use when I am out in the middle of nowhere, or when the internet goes down.. which for me is an almost daily experience for me since I live in the middle of nowhere.
When the internet is down, I find it to be the perfect time to write without interruption. I always open the TBS only to realize I can't use it.
I love writing in TBS. I love being to write several versions at the same time, and I do not necessarily use the synonym feature when I use the TBS at all. I simply find TBS easy to write into, and it suits the way I write and work.