She was an economics student (season 5) so she could work for GNB or another company in which Ted will design a building/work for.
Their share of the same hobbies
I could really care less about getting to know the mother. This show is about all the characters for me and the title of the show could imply to all the male characters. I've enjoyed seeing how Marshall and Lilly met and ended up together, and even though Robin can't have kids the changes Barney has gone through over 8 seasons has been fun to watch.
I might be the only one that's excited for a 9th season, but I think that will give Carter/Bays a good way to tie their show all together and end the show how it should, instead of stuffing 10 episodes full of things.
I'm not saying you guys are wrong about wanting the mother, and I wouldn't hate seeing her on screen for some episodes. I just don't think it'll happen. It would be like seeing 12 episodes of Battlestar Gallactica colonizing Earth, it's just not that needed.