Hm, I am actually not sure what kind of searchability the OP is talking about. Of course you can't have random apps clutter a search in contacts. What happens on Android is that an App can specify that it is ready to answer certain types of searches. In certain situations, the user can choose the app he wants to search.
Lot's of useful applications for that come to mind. Do I have to ship a spell checker with my app, or can all apps share a spell checker? And so on...
Of course, yes, it could result in utter chaos, with 100reds of apps to choose from for a particular search. That kind of problem is really overblown, though, just like the multitasking problem. If an app annoys me or sucks too much energy, I just uninstall it...
That goes for pretty much any kind of "developers are going to abuse feature X" argument - annoying apps just get uninstalled, end of story.
There is a problem of potential sneaky apps, I guess. That is also existent on the iPhone, I suppose (don't they do their own user tracking), but of course there is slightly more potential with background tasks. Some common sense can eliminate most of the problems, though (ie why does an ebook reader need my GPS coordinates? Skip it).
