Of course, mounting isn't required for playing video DVDs and is not possible for audio CDs. A/V players are designed to use the raw devices, though for DVDs, one can mount them (they're bridged UDF/CD9660 filesystems) and use players with the VOB sets, if desired.
Block devices are only required for mounting filesystems, and will degrade performance if used for other purposes. Raw is recommended, and, for some operations, such as formatting partitions or burning optical media, raw devices are required. e.g.: compare the results from these two commands, run on an unmounted partition:
# time dd if=/dev/wd0g of=/dev/null count=10000
# time dd if=/dev/rwd0g of=/dev/null count=10000
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