Why doesn't it need 40MBps? If 40MBps is available to it why not go for it? What's the harm in pusing the bit-rate if you still have ample room for uncompressed audio and HD extras? I will dig this, up but I'm pretty sure a couple of the AVC titles breach the 40Mbps limit.
Well, if there comes a point at a lower bitrate when you become perceptually lossless then more bits don't add anything at all. And who wants uncompressed audio? That's just a waste when there are lossless compressed formats available.
I can't say that there is no potential benefit to using a higher bitrate with AVC/VC-1 that might be afforded by BD-50 over HD DVD, but BluRay was originally targeted to use MPEG2 compression exclusively and only added the advanced codecs because of lobbying by the studios (most notably Warner). If they thought that they could achieve sufficient quality with MPEG2 @ 50GB it's not that much of a stretch to beleive that sufficient quality could be achieved with the advanced codecs @ 30GB.
I'm not saying this is the case, but until someone does a double-blind encoding test using mutiple different test clips of losslessly compressed material using AVC/VC-1 at escalating bitrates, it's impossible to say it isn't either.
I'm pretty sure the key differences between the 2 formats are going to be studio support and price.