Who are these "spartans" you're talking about (and where can I look them up)? Obviously not the real life Spartans from antiquity...
And you can also compare to the multiplayer version of the Republic Commando, which can only carry two weapons (each attachment for the DC-17 counts as a "weapon") and three types of grenades (the fourth type isn't even available in multiplayer). What is this "fifth" type of grenade? There's only four in the game: sonic detonators, thermal detonators, EC detonators, and the Diversionary Flash detonators (SP only).
The ammo capacity of your guns is also smaller in the MP portion of the game. While more limiting, overall the MP presentation is probably more realistic for an individual commando (though carrying three attachments would seem logical enough, plus a pistol, among a squad it would make more sense to break up the weapons between them rather than giving each person everything).
Bacta of course works differently in MP vs. SP. In SP if you're not using a station (which takes time to use), you have to have somebody on your team revive you (which also takes time and only brings you back to half health). In MP you just run over a bacta tank and it instantly restores you to full almost instantly.
See the link to "BOOT CAMP" in my sig for further details.