Originally posted by: dumb_kid

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).

Plus extra features like PIP, video featurettes and extra audio tracks. It could mean the difference between have just a DD track or having DD and DTS, plus three commentaries and a PIP feature. More space is always better.