Why? Because PC port will receive a boost in graphics and performance. On top of that PC crowd gets to enjoy MP free of charge. That's why. Dunno about you Pal, but I'd feel a little upset if I purchased a product and then found out that I was ripped off because I paid more for less.
While there's no alternative to your performance downside, other than Xbox owners moving to a PC of course, I can very well get behind and support everyone for wanting free MP on every platform, unlike all the corporate apologists who claim "well, you paid them all these years, so doesn't make a difference if you continue to get ripped off."
Where they're completely disingenuous is the fact that entire lineup of IP's owned by them (read: OWNED BY MSFT unlike SONY's situation with SFV), like KI/GeOW: UE/Forza 6/Forza Horizon 3/Halo 5/Halo: MCC instead of an occasional Halo 2 Vista or a Gears of War GFWL, can be played by the PC folk for free and the Xbox userbase has to cough up $60 a year. Another complete deterrent for Xbox purchases or ownership would be any future MP focused game being locked behind gold for MP on X1, but free on PC and releasing the same day on PC & Xbox One. When you say this, it will get spun into "WOW OMG CONSOLE FANBOY WANTS TIMED EXCLUSIVITY GRRRRR", but your real intention is just free MP and happy that more players are getting to play that game since it's headed to PC. Let alone PC/X1 crossplay, which is an even worse value proposition for an Xbox user paying for MP.
Another popular argument is "Oh b-b-but PC users don't have Games With Gold, so it's worthy for you to pay". That's entirely the point. Feel GWG is "worthy enough" to move subscriptions, like PS+ did with the PS3? Then don't lock MP behind a paywall.
Granted, the LIVE subs bring in free money (tons of'em), it would indeed be a game changer vs PS4 going forward. Since exclusives are eliminated (or they seem so for the past 3-4 days, with just Quantum Break confirmed officially) from the equation now, go for your mainstream revenue generator. Multiplats. Imagine the Destiny, CoD, BF, Fifa, and the first party big hitters like Halo, Gears, Forza blowing up in numbers if MP went free. While it may not get closer to PS4 numbers in consoles sold, software won't be far behind. All of this w/o spinning off the Xbox LIVE sub, and just by having GWG as it's forefront value proposition, just like the PS+ IGC once was for PS3.