Well, it can slightly reduce loading time. However, reading from the DVD / blu-ray / HDD is not the only time consumming operation that can happen during those loading screen. In fact, it is generally not the longest operation.
After reading the data from the disc, the game still has to :
- decompress it (data is generally compressed to reduce used space)
- rebuild cross-references (between textures and models, ...)
- ...
Those operations generally take more than half of the loading time. There exists some techniques to reduce this time (streaming, ...) but they may not be applicable to all types of game, and they can be difficult to implement.
Game are allowed to use some space on the HDD as a cache of blu-ray / DVD (both on Xbox and PS3). This is not that much used as installation (either via the Guide on Xbox, or via the game on PS3) is generally a better option (as the cache can be cleared automatically by the system after exiting the game).
On some specific games, loading time can be drastically improved however. Gran Turismo 5 for PS3 loading time can be halved by using a SSD.
