[1] Given analysts use AdMob metrics to figure out market shares of various mobile phones
Google might be able to replicate the Adbob business but it would take time. Admob + Google's existing strength is a big head start.
I'm sure they also wanted the talent. I've met one or two of the AdMob guys and they seem pretty sharp.
The talent angle of course makes sense, they are both pulling in top tier employees and removing top tier competition.
scenario b: spend cash you can easily afford, have an immediate cash flow return, get a solution that is known to work, eliminate competitive threat without pissing off anyone in the process while ingesting talent at the same time
it's fairly easy to see why GOOG chose the latter option.
It's a bit of a chicken-and-egg scenario (devs will not move until the advertisers are there, etc.) and much easier to just buy them outright.