Exactly how many movies do you know of that use all 30Gbs, much less 50Gbs?
Here are just a couple:
Apocalypto - 34.6 GB
Casino Royale - 35.2 GB
Close Encounters of the Third Kind (with seamless branching) - 37.3 GB
Face/Off - 35.8 GB
The Fifth Element - 37.7GB
Flags of Our Fathers - 35.3 GB
Hellboy - 39.4 GB
...
for a lot more Link
All of the ones I listed were AVC encoded
Even so, it's debatable whether there's a notable quality difference, but more space is more space. Plus BR has higher data throughput which is also nice to have. The extra space is very nice to have for extra long movies (LotR) or seemless branching(dare to dream SW).
I don't get the die hardness of fanbois on either side... it a fricken video format -who the hell cares. I'm excited that BR is very close to becoming the standard, so I can just enjoy movies not worry which format to choose because Studio X supports Format Z. Here's hoping Universal and Paramount can join the fold soon.