The bacteria doesn't need to adhere to the rock - it just needs to be in the water that gets sucked up along with the rock when whatever cataclysmic event causes the asteroid to be formed. And certainly if earth bacteria are typical, reanimating them is just a matter of sticking them somewhere warm and light (although in lab experiments they've also got food in the form of agar gel, I guess).|
But yes, they'd need to be lucky to impact on a planet which wasn't being boiled by the star or drifting cold in the emptiness of space. An atmosphere would probably help too, so it also needs a molten ferrous core (I think). But given the number of planets apparently out there, such host planets must not be entirely uncommon.
Biggest question in my mind is the cataclysmic event that creates the asteroid in the first place. I can't conceive of such an event that wouldn't result in great heat and pressure, so the bacteria involved would need to be extremely adaptable to survive.
Cubby didn't know how to turn off sigs!