I would think the biggest detriment of only 1 card slot would be having to remove a map card and install a blank SD card anytime you wanted to save a snapshot or make a recording on the 598ci HD SI Combo...
Snapshots and Recordings are generally performed on the water for saving interesting finds .....so if a map card was being used (and you will want a map card) ....you lose your map data (and only have the base map) in order to make a Snapshot or Recording...
Exporting waypoint data to a SD card is generally done "after" the fishing day is over... So 1 card slot for this is not a big deal...
Rickie