So why does multiplayer need balancing and single-player doesn't? If you don't want to be underpowered compared to everyone else, then don't choose an underpowered class in a multiplayer game. Exactly the same as in single-player, no? If you're looking for extra challenge in single-player, you gimp yourself in some ways. If you're looking for extra challenge in multiplayer, you gimp yourself in some ways. No? Do you not ultimately play the way you want in multiplayer as well?
I think balancing is needed, because gimping myself intentionally means removing tactical options. I don't have any examples for D:OS yet, but for example I went through UFO: Alien Invasion on hardest difficulty pretty easily. Smoke in the tactical battles is very powerful. Now, I could just not use smoke, but this is removing a big part of tactical options (protecting civilians with smoke, advancing under cover of smoke, etc.), which results in reduced tactical options and gameplay. I could also not research the coil gun or the electromagnetic rifle, again removing a tactical option to make the game more challenging.
In D:OS I suppose I could not hire any companions or henchmen, removing the tactical options brought by a bigger party. I could also not use higher-tier gear instead of thinking "If I put this strength+, intelligence+ gear on this guy, he could suddenly use both warrior and mage heals and become a better healer than my current acting healer, freeing the acting healer to do this-and-this." But in my opinion I might just as well not even play the game if I have to start meta-gaming to keep it interesting.
