This could be an interesting discussion, but I'm really baffled that this comes from someone whose introduction to AtariAge was a Disk Manager, for OS X only, that AFAIK doesn't do anything that TI Image Tool wouldn't already do. So why did YOU start a completely new project instead of joining an existing project?!
For me the TI 99 is about the community. Everybody has his or her own motivations on why they deal with an obsolete system: Some like to tinker, some like to play, some want to finish what they left 30 years ago, some cherish their childhood memories, some want to learn new stuff, some like to have a second go at it with much broader knowledge now, some are obsessive collectors, some are even in it for the money -- and they probably all just want to have fun. Any reason for tinkering with the TI 99 is as good as anyone's else, and I won't judge people by it.
Having said that, I think cross-development is simply the most efficient way of getting results onto the TI 99. Of course these results might just as well be native tools -- think of it as boot-strapping. Didn't Texas Instruments use mini computers to write software for the 99 as well? If we design and use new hardware for the TI that includes today's state-of-the-art technology, why shouldn't we design new software using today's state-of-the-art as well?
I don't own a SAMS card, a hard-disk controller, or a Geneve, even though I've been on the lookout for years. I can and do appreciate the technical prowess that went into their design, but these options are just out of reach for me to build new software.
For me xdt99 is, among other things, a means to my own game (that I started work on about two weeks ago). Once in a while you get side tracked a little, like by Rich and his enthusiasm for GPL, but that's the fun part of it all. And we all here do use other people's tools regularly -- there probably isn't anything that Tursi doesn't have covered.
So I can't see an issue here at all -- the community as a whole is always exploring, looking for new hacks, based on the previous work of others. What else could you wish for?