Well, for a Fluther app to work, an API would have to be provided granting access to all the juicy bits behind the scenes. I don’t think that exists yet (if ever).
But I could certainly see where some new functionality, not available on the web, could be surfaced in an interactive app. (Much like Twirl for tweets on Twitter.)
What if, for example, you wanted to monitor everything ever said by @waterskier2007 b/c he’s your fave guy? Can’t get there today on the Fluther web site. Or what if you wanted to limit Questions/Answers shown to you from only people you’re interested in (or that meet certain tag-word requirements). Not something I’d want, just saying. Plus you’d be able to setup desktop alerts and whatnot (although Fluther’s RSS and email aren’t too bad at that.)
Of course, the Fluther gods can (and do!) add new functionality to the site, and that’s probably sufficient for most of us. But I can see where an interactive app might be worthwhile. And to be clear, I’m in full agreement on the whole data transfer/bandwith dealio already mentioned… but that’s a problem that’s handled with throttles and data pagination, if done properly. ‘Course, it’s that kind of thing that is partially responsible for keeping Twitter on the ropes.