How about "Um, survival?"
If humanity is going to survive for any longer period of time, we have to make a self sustaining colony somewhere other than this mudball. Outside of this solar system would be better, but I can accept another planet in this one as a good start. Even a (semi-)independent moonbase would be a decent investment as a training/prototype/testing facility.
I've heard people argue that "we have plenty of time, we should fix earth first", to that I say:
Off the top of my head I can think of 3 different potential civilization-killer level events that we're overdue for, in the sense that the last one was longer ago than the average time between events - Big asteroid/comet (think dinosaur killer), Supervolcano(es) (enough dust in the atmosphere for a year or three with no place on earth where the temperature goes above freezing) and Large Scale (natural) Climate fluctuations (the last few tens of thousands of years has had some of the most stable and hospitable weather for as long as we can determine, according to some sources; if large scale agriculture fails, how far behind is the rest of civilization? How large a percentage of a society needs to starve before it collapses?).
It is not a matter of "if we go" (maybe not Mars, but somewhere), but "if we survive". If you accept that we should try to survive, it becomes "when we go".