Endace Cards are recommended for those customers with traffic over 45Mb/s reaching the monitor machine. Note that the rate at which Symantec DLP can detect in real time is significantly lower than what it can aquire from the wire. However, not all customers demand detection in real time, and Network Monitor is able to queue traffic well ahead of how quickly detection can process it.
Additional reasons for an Endace:
You can obtain Endace-like performance with a NIC if the OS is Linux. The native capture layer for Linux monitors in will provide an alternative in many circumstances.
Overall, though, if a site is willing and able to split, filter, or otherwise condition the offered load such that one or more monitors receive clean, low-rate feeds, then that is a perfectly acceptable deployment scenario.