I do not lose a port anyway: The OPV is on the base of the 1st stage reg.
But, it is stupid to put that OPV on the 1st stage:
1) In case of any level of free-flow from the OPV on your back you cannot see it. Maybe "your buddy" ..or maybe "you can hear it" ..& maybe not.
2) The OPV is required just as long as you use the Xstream 2nd stage reg. If you use, for example, APEKS 2nd stage on the Xstream 1st stage reg. you do not need that OPV at all.
3) I have 3 sets of Xstream without OPV. In that case, the hose has the OPV on the connector to the 2nd stage. In that case, if I buy the MK3 with OPV built-in on the 1st stage, I need to mix 2 types of 1st reg. & 2 types of hoses in my tool-box. which leaves me less redundancy on equipment.