No. You can gather Omega Marks on Defera solo, just not as fast.
Ahhh...this is my first time at Defera. Very cool. It's nice to do ground combat without the confusion of my landing party around. Also feels like a real MMO* with other players running around helping each other. I played for about 8 months, then quit, and was daunted by trying to relearn the game, but I think I've got the hang of it again.
*Of course the level of the MMO experience is up to me. I just don't like teaming up with others (except for the red alert), but the random appearance of a real person is very cool.