good questions
1) I believe there MAY be 5 to 10 people at level 15, and i am not one of them. obviously it takes more than a year to get to level 15, and MOST of those points are spent simply getting promotions (if you get it in a year or so, i'm at 2 years and not quite 15, but i spent 10,000 on resistance). it just makes sense to earn a permanent bonus
2) these are significant because they are PERMANENT. No, they aren't worth it enough to get level 15 in all careers, because that would take you 12 years to earn. and by then, there will be new stuff to focus on i'm sure. so why not?