Alpha and beta software is in a state of flux.
As arantor said there is a certain level of debugging that s expected from people who are running test versions... And it would be useless to try and develop a theme for 2.1 yet, since, in alpha it is possible to completely drop or rewrite anything.
Well, I always considered 2.1 quite "stable" theme-side because we really didn't change much of the background (of course in the early steps we changed few things, but nothing that would require a total rebuild of the theme, rewrite of certain things, but not completely. Of course it may become a bit tricky to keep track of changes if you are not using a local repo...that's true. And of course it all depends on what you want to obtain from your theme, if you just want to tweak the css it's one thing, if you want to redo it almost entirely it's another and the two options require different approaches).
As much as I like the availability of the code on github, it does present a problem because general users will nstalled it and expect support. However, there is no support for alpha and only limited for beta software.
What you are missing here is that we are supporting our product.
We are not giving support to the users of our product.
In that phase it's SMF that needs support and feedback from people installing it and using it, this is support that *we* need.
Building a new theme may let AllanD discover a bug somewhere, or may allow him to suggest some trick to the template that would improve the theming experience, or whatever.
Have users that create mods for devel versions provides feedback to improve some aspect (we introduced tons of hooks, but do we really know if they are *really* useful? No.). Of course is a possible waste of time, but that's part of the game (all the time we "developers" are putting into our code is potentially a huge waste of time, but we nevertheless continue to do it).
We have warned people (at least I think), so as long as they are aware of that, play with a development version (and anyway the current 2.1 doesn't receive many commits) is fine. And if they don't know or don't understand that, they will learn at some point.