Theoretically, I'm in favour of putting swathes of SD content on BD, for the gains in storage capacity & audio quality, as well as the possibly marginal gain in video quality. However, a lot of the preceding arguments for this ignore the fundamental point that the studios can't be counted on to do it the right way - quite the opposite, in fact.
The studios aren't run by audio/videophiles who want to put out the best quality product possible; they mainly just want to shift as many units as they can & as cheaply as they can. Naturally, they'll simply transfer the same old SD tape masters to BD, with no rescanning & likely, especially with TV shows, no audio upgrade. What's more, as has already happened with at least the Bee Gees BD, they'll arbitrarily crop 4:3 ratio material to 16:9 - you can count on it.
If you take a look at the German Amazon site, you'll see that reviewers are kicking up a stink about the fact that seven Blu-rays have been released of Hugo van Lawick's beautiful wildlife documentaries, but they're all in SD, from ageing tape masters & with lossy audio. All of his documentaries were originally shot on 35mm film &, with the right amount of care, could look as good as more recent releases, such as Planet Earth. None of these BDs are flagged as only containing SD material, but they are being sold at full price - very naughty. This sort of practice would, I'm sure, become only more commonplace.
The aforementioned Bee Gees BD was one of four such discs released by Eagle Rock Entertainment last summer: Bee Gees - One Night Only, Scorpions - Moment of Glory, Rolling Stones - Stones in Exile & Pink Floyd Classic Albums: The Making of Dark Side of the Moon.
Does anyone know of any other non-public domain SD BDs?