I thought that this was quite an interesting development for the Marketplace. It's a step. Just a step. What would be really useful was if there was a scale on the instal graph so you could get a handle on what it meant. Perhaps that will come.
But why do we need this data? Good question, I don't know really. Sometimes it helps to know if the installs on an app back up wild claims about what it does, although from that point of view it would be better to have a graph of how long users had the app on their device before deleting it.
Anyway, a step. Nice one Google. More please!