Documentations found on web says, MLLP V2 (release 2) is a requirement for all Message Transport protocols used to transport HL7 Version 3 content. It seems that MLLP release 2 is used mostly with HL7 version 3 only.
Can/Should we use MLLP release 2 as transport protocol to transport HL7 version 2?
Why MLLP V2 is a requirement to transport HL7 version 3 content?
MLLP release 2 is based on version 1, but goes further by adding defined ack/nak messages, so that the data exchange gets reliable. Now the sender knows if the message went thru or he should act e.g. by sending the message again. If you use asynchronous communication in HL7 V3, the sender needs a way to know if the sending was successful.
As long as synchronous communication with the respective acknowledgement messages is used, it is debatable if there is need for the use of MLLP release 2 in HL7 V2.