A few reasons why this might be a good idea:

1. Extra testing of getByteArray, since all getSigned/UnsignedLong would now be using a similar codepath
2. One place where all the BE/LE/MSBF/LSBF bit twiddling stuff goes, making it easier to optimize in the future
3. The existing LongConverters have some code that is potentially slow (e.g. reversing bit/byte order of byte buffer, lots of sign extended/unextended, etc.). The getByteArray method is a pretty efficient way to get any amount of bits in to BE MSBF order, which can then be quickly converted to a Long with a few shifts and ors.