What if you have a short-term consultant on your team? What if you're the team lead for 

just this project, and there will be a new team lead in the next project?

Should you work on growing people even in those situations?

My answer is an unequivocal yes. (unless we're in chaos mode).

People who come across my path , even for a short time, will get the same

amount of respect, expectations and challenge from me, as if they were there forever.

I will try to always leave people who I've led, better off than they were, in terms 

of new skills and challenges.

It is in my personal integrity to do so. If you conduct yourself this way,

you'll find that people will want to work more with you, and will remember you 

even years later, no matter how short the project was.

You never know when you're going to meet those people again, and 

you may find hidden treasures are laying down in your future path, just because 

a few years back, you did the "right thing" with people who didn't expect you to.

Show a personal example that even in the short term, you do not give up 

on quality (of people, of leadership).