I honestly don't think that they will be able to have a career. Being a princess is a career in and of itself, and above and beyond that, what kind of meaningful job could they do, when they would likely be surrounded by the press all of the time.
Not to mention, that as part of the Royal Family, they represent the UK, and by working in any specific sector or company it might seem as if they arelending credibility to that sector or company, or playing favorites. On top of that, as a representative of the UK, they shouldn't be seen to be dealing in anything overly business like or political because it might appear as if the state were sanctioning it.
Further more, who would hire them? If they do a bad job, do you think anyone would want to fire a future queen or princess?