I don't know why the hell studios think thats a good Idea as most people I know including my self will not buy these things, it's either a full season or nothing, so useing sales of these dvd's (something like 6 disjointed episodes for $20) to measure how well a full season release will do is bullcrap
Because the single disc "sampler" catches the eye of the "causal fan" of the material, the ones that might balk at paying $30-50 for a season set are more likely to pay $10-15 (usually less than a new release movie) for a single disc with 3-5 eps on it.
On top of that, you
might not go for it, but experience has shown that enough "hard core" fans will
to make it worth their while, added on top of "casual" fans.