The reason I took those is because the information I recieved about that final choice suggested that the success of the defenders relied upon who was left behind.

Spoiler Alert, click show to read: 
For the final choice, it's all based upon the characters relative 'defensive ability', it seems; some are high (Jacob, Garrus, Miranda, Grunt), others are low (Jack, Mordin, Tali, Legion). If you leave behind enough of the 'high values', you're more than likely going to ensure that those with them survive. This link explains it well.

I didn't have Jack's loyalty, wanted the 75G for the achievement, and was really cagey about her dying so I ended up taking her with me at all points up to that part. At the final choice, I decided to have Mordin head back to the Normandy with the crew (cos he's squishy too, loyal or not) and then take Tali and Legion with me. Samara, Thane, Garrus, Grunt, Miranda, Jacob and Jack all stayed behind to hold the line, and it worked out.


Unfortunately for Tali, I feel her weapon combination of Shotguns/Pistols just doesn't really do her any favours. She needs a weapon with a higher rate of fire, or more health to offset the fact she prefers to be up-close with the shotgun. Otherwise she's simply a weaker, more tech-orientated version of Grunt or Jacob.

Legion's actually pretty decent, his unique sniper rifle sorta made my jaw drop at how powerful it was, but I feel that the fact he's geth and he comes into the game so late doesn't really give you much scope for relating to him as you might Garrus or Miranda, for example.