I previously would have said yes as PM used to give LHR Clubhouse access (when in Y or W), but as that benefit has been pulled... meh. (CH still available in a handful of US locations.)

PM also gets you RUCs (though at the cost of the 20k RDM). If you flew the NY/CA TCONs, these would pay for themselves in terms of upgrades, though they may still have value on other routes where you cut to the top of the list.

20 hours to get PM? I probably wouldn't, unless you enjoy being in planes or going somewhere interesting.