. . . and if you've tracked it long enough, it has already either hit or, more likely by far, missed by hundreds of miles. The bigger object you find, the less effect you can have on its trajectory with a given bopper--and the sooner you'd better bop it, so the effect accumulates over time.The conclusion is that you do well to detect early, and to refine your estimate of trajectory rapidly when you do detect. Does Hubble's different perspective help, and would a deep space probe be enough better to justify its cost?
They don't use Hubble for tracking asteroids, and I'm not sure it could do a far better job than a smaller, ground-based telescope. The first order of business is to find the darn things, and for that you need more eyes rather than better eyes.
I'm just guessing here, but I expect that the biggest advantage to a space-based telescope would be to give you a long baseline for parallax measurements. If that's right, the goal would be to get the space-based telescope as far away from Earth as possible. Perhaps a Moon-based telescope would be a good solution.