Are you a software developer?
Among other things, yes, although it isn't currently my main professional activity.
I am and I can tell you that trying to engineer for a moving target is both difficult (at best) and a real waste of time and engineering resources...things at the core engine level are changing all the time...there is no guarantee that ANY engineering done now could even be used on the final 1.0 product.
That's close to tautological, if one assumes that this part of the product would be in flux all the time up to a 1.0 release.
However, leaving that part of the product in flux for so long is bad software engineering, as you well know.
I've had to work as a third party developer against software like that (and when I write "against", I mean "against"), and it's something that I'd rather not do, but hey, it paid good money to fix the problems creatively.
No, working on the full SDK -AFTER- the final version is locked down is the only thing that makes sense...which is why the Lightroom engineers are taking this position.
It only makes sense if you assume that SDK and API are perfect in their first released incarnation.
If not, you'll end up with patch releases and API changes even less consistent than those of Microsoft Office, or you'll stick to an API that most likely has glaringly obvious faults and problems but which you'll keep around for ages because you want backwards compatibility (like the legacy Windows 16-bit API).
This is to the detriment both for third party developers as well as end users.
Frankly, Lightroom is really at the pre-alpha stage. The only reason Adobe calls it "beta" is they figured most people wouldn't understand a "pre-alpha" concept. This is software engineering with all the guts exposed. And until they stuff the guts in and sew it back up, there's no reason for others sticking their hands in.
Well, it's not quite pre-alpha, because it appears to be mostly stable both feature-wise and in terms of data corruption and runtime.
Also, very little of the guts are exposed, as opposed to e.g. the GIMP, where you can feel free to dig around in the guts all you want (ewwww).
As David (drm) mentions in his response, yes, there could be a beta API available by now, and it could be used by third party developers to provide presumably valuable feedback on the API.
The SDK could wait a bit longer, of course, since that in itself depends on the API.
But who knows, maybe there will be a market for a third-party API wrapper ...