It is strange jriver have gone down the memory route given their position on jplay and that any bit perfect player cannot be improved upon.
Their comments are:
"Sound Quality
JRiver is unaware of any test that shows a sound quality advantage to memory playback.
The option was added due to popular demand."
Full description of the feature here.
Plenty other reasons already you might want to upgrade from 18 to 19 - see here. Upgrade was less than $20 to those that fancied trying it.
ps Plugplayer is pants compared to JRemote.