For sales ask sunrise, but iirc about 350+ have been sold orso.. this number is divided between v1 and v2 moonsounds (diff being the place of the output connector and the extra slot for RAM).
Individual tunes also count as Moonsound content.. there's some in the MRC database, and ofcourse Moonsound is used here and there in MRC challanges. 
*cough*
For more activity ppl need better software I estimate, MBWave/FM have worn out and we're still waiting on Meridian. In any case Moonsound and tR aren't related. It could very well be that many Moonsound owners have a tR (wouldn't know really), but that's just being luck then I guess.
Other than that, G9k has the disadvantage that it's something essential in a game or demo. No required G9k? No game or demo. Sound/music can be seen more as an option. No Moonsound? Still visuals.. just no music (or an FM/PSG version if you're lucky) etc.
~350 is a nice count, but in the light of the entire msx community it's not really a lot to me... of course, emulation makes a difference (I've been running several moonsound using demos on blue, never tried any gfx9000-related though (is it supported? I wouldn't know even)). It's probably a lot more than the # of sold gfx9000's.
You're right about Moonsound being optional (allthough Sphere and Wings are kinda missing something without one 
), but that brings me to another question... is the GFX9000 too different from the others to make a game that can use it optionally (e.g., like some MSX1 games can look better on MSX2)?
Where's the FAC when ya need'm most...
Well, when using P mode, you have something like sc2/4, but then with full-detail tiles rather than having sc2/4 limits. Also, there's something like 16 fullcolor sprites on a line, compared to 4 on sc2 or 8 on sc4 (which are monocolor per line even!). So I *guess* uplifting sc2/4 games should be doable.
There're also modes like sc5, you gain sprites again, and you've ofcourse more palette options.. so even that should not be impossible to do.
Overal you also gain speed ofcourse, but in case of these games being compatible with MSX1/2 your speed will prolly be aimed @ MSX1/2. If the G9k is only a compatibility update then you prolly can't do a lot more except upgraded sprites and colors.
Question is: who will do all that? 
Making games is hard enough already, that's the major issue. Back then the G9k was marketed as being an easy card for games. While that might be true, it doesn't make games easier. Only the output to the screen got easier and faster, but making the actual game is still a lot of work.. I mean, an RPG is still an RPG.. e.g. 'hard'.
It's somewhat the same in the Blitzbasic community. That language is also marketed as a game-language, and a lot of ppl start asking questions in the beginners section like 'how do I make a scrolling map?', 'how do I make Doom3?', 'how do I make a ship explosion?'. And if they post some of their code it's truly unstructured crap where every var is a global orso Anyway, point being: a game language, or a game card, doesn't excist. it's still all hard work with normal code. The only game'ish thing (in both the G9k and the Blitz situation) is gfx-routines.
So, is it that surprising that there's hardly any G9k content like games? There's a certain magnet flying over G9k games that lifts those projects up into the air. When they're high enough, the typically 2-3 teammembers creating the game can't reach it anymore, and gone is the game.
And oddly enough, last few years ppl seem to have moved back to the MSX1. (MSXDev) .. it's not something that helps the G9k's popularity (or lack of..?).
I don't think the move "back to msx1" is so odd. We have internet now, and the contact between the formerly much more localized communities has become much more intense; it's now more a worldwide community, with the exception of the ever difficult to communicate with Japanese. Spanish and Brazilian MSX users are very active, and they're often MSX1 users, which is why I think we've seen this move. The Dutch community is much more MSX2 / MoonSound (and Turbo-R? ) oriented
I think, in Brazil, desiplus is not cult 
I meant that potential G9k users are now doing MSX1 games. XL2s was/is working on some G9k platformer, instead they joined MSXDev '05 and '06. We (me+edwin) have G9k's and did some minor things in its conceptphase for G9k, yet we joined MSXDev '05 and '06, TNI has G9k's and yet, they joined MSXDev '06. While 2/3 of these names cancelled, it's just to show that ppl moved towards MSX1 while they should actually be creating G9k content.
is the GFX9000 too different from the others to make a game that can use it optionally (e.g., like some MSX1 games can look better on MSX2)?
The difference is that it's far more easy to make sound an option as graphics. For sound you can be lazy and just have the user opt between sound or no sound. For graphics, and especially Gfx9000 gfx vs V99x8 gfx, you will have to create all graphic content twice and all routines you need to properly show those graphics; there is no 'be-lazy'option here 
