1. How exactly are these spores of life getting into asteroids and being pinged around the Universe? The mechanism seems absurd to me. Earth is teeming with life, but outside of our explorations I believe none of it escapes our planet at all.
2. The life that would reach us would be already specialised for whatever environment it came from, and I would imagine would result a lower amout of variety, since it's already a successful species. But on our planet we have a massive amount of variety, which I believe points more to a bootstrapped start where competition was less intense, and odd things could develop, which were later outcompeted (see the Cambrian explosion).
Disclaimer: Not even a biologist, but I'm more willing to accept that life started and evolved here. Of course that might all be cast in doubt if we detect masses of life on asteriods or metoerites, but the first attempt at that seems to have come up with nothing so far.