Unless they plan to do more with the multiplayer this time around, then I really don't think it's going to be a standout feature for someone to buy MAX for.
Not sure what their plans are for it, but as far as the multiplayer that was done for Classic, it was very barebones, and I believe deathmatch and TDM were the only modes available. No voice communication, no round based matches (if I remember correctly), no weapon selection or loadout assessment at the start (you just placed weapons around the map) and I believe the player limit was 4 per side(?). I don't even think doors could be opened, but I can't remember for sure (though I know for certain this wasn't possible with FPSC's multiplayer until hockeykid got it to work in Project Blue or Fenix Mod).
It reminded me more of 90's Quake or DOOM multiplayer than a more modern multiplayer experience.
It was a shame, because I agree. If more time was invested into multiplayer this time around, it could be a very appealing selling point for the engine, and I would probably focus more on making a FTP multiplayer game over a single player one if that were ever the case, because even though Classic's multiplayer was limited, it was still a blast playing with others in a map you built yourself. Done right this time around, it would be pretty incredible.
But yes, standalone would be a must, for sure!
Once a wolf, always a wolf.