I can only speak for myself on this topic, but if you go solely on my opinion, then I think there are actually tons of great customs out there, that never get made public because the owners want sole use in leagues. That's great and understandable, but it's really a shame that some really great work never gets out there to be seen. Can't there be a master list of "exclusive" trucks and what players have the rights to them? Or maybe when new versions of trucks are made the previous versions are released? Obviously, there is a segment of the MTM2 playing population that wants to see the work, and not necessarily in a drag league environment. There has to be a way to put work out there and still have it for league use.
For my own part, I don't see the point in putting in hours of work on a truck and letting it rot on my hard drive as a "personal" thing. I do it as a creative release, for myself. Even as comments on customs have become scarce, I still somehow get satifaction just by seeing the d-load count go up on the Trucks page. People like my stuff enough to want to use it. That makes me proud.
Also, seeing as I don't run leagues, I have no concerns about "exclusivity". In fact, if you really pay attention, you'll notice that only one of my trucks, Driven to Destuction, is "my truck" as it has my name on it. All the other trucks exist as to be driven by fictional (for the most part) characters, which means anyone can take a drive and not be stepping on toes. Heck, you can drive DTD if you want and pretend to be me. You may just like the paint, of course, and that's fine, too...actually, it's better because you're a sad person if toy want to pretend to be me.
If you want to see more customs, my suggestion is to speak up and give some kudos when you see trucks you like. If guys realize that customs are getting some love, they'll be more likely to make more. Me? I'm gonna keep making them, because they're way more fun to make than replicas!