Many of those uses already have specialized apps, which do search within their context-relevant data, in context-relevant ways. iPads in medical environments are a perfect example. Why, if you're searching for symptom X, would you want to find emails with X, text messages with X, and that image which just happens to contain X in its filename?
Essentially every laptop / tablet / whatever-PC in use in the medical field works similarly - they use one application which handles all relevant data, and it doesn't search outside that. And this is on a PC, which is essentially wide open to application interoperability (if not as easy to do). Developers and users have already apparently decided that it's not as important / desirable as many would like to believe.
Granted, there are cases where this is useful, but it's a fundamental iOS / Android(/Unix) design choice. Android favors interoperation through APIs, iOS favors using one application for the job at hand.
Why, if you're searching for symptom X, would you want to find emails with X, text messages with X, and that image which just happens to contain X in its filename?
You could very well want any of those things searchable for symptom X if the email, text message, or Image has relevant details for symptom X. Perhaps the image is an X-ray, the email is a diagnosis from a colleague, or the text message is relevant data about the diagnosis. The very fact that you think they should have to load 3+ different apps to find all the data relevant for symptom X or purchase a high priced propietary app that somehow bundles email, imaging, documents and sms all into one interface shows a lack of vision. Just because every medical device in the field works that way doesn't mean it should.
But without it being in database X, it's (very) hard to know the context of those sources. Without context, you're more likely to get irrelevant data which just happens to contain X. Unless your machine has no irrelevant-to-your-current-query data in it (unlikely). And in that case, the whole device is effectively a one-trick-pony, significantly less useful than simply being bound to a single application.
Know of a search API which properly handles (every possible use of) context? Without that, people distrust searching because of irrelevant results. Google does well because of PageRank; as far as I'm aware, nobody has arbitrary-context searching which is effective, because it's such a ridiculously complex and/or processing-intensive problem. Such a filter would probably be integral to passing the Turing test, so we'd probably have heard of it if it existed.
I think your inflating how useful having it in a database is. The context comes from reading/viewing the content. A database isn't automatically going to fix that. If you need information about "Lupus" (yeah I know it's never lupus) and it's about Tom your new patient. And you know you getting either an email or SMS from Greg the specialist on the way to the office and all you have to do is hit the search button and type in Lupus, Tom and Greg to get that information then your phone has just made your life easier.
The context space is much smaller on a device like a phone and it's quite possible to craft a query that finds what you need faster than finding the app, opening it finding the search option then typing it in.
On my Android phone I love the search feature. It's a lot like having quicksilver on my phone. A single button that gets me access to everything.
On a simplistic smart-phone, sure. But most beasts-of-burden nowadays have at least a few gigabytes of storage. That's not any smaller than a PC - few applications have that much data (aside from games), even in the medical field.
I may very well be inflating things, but the instant you mention "craft a query" you've lost 99% of people, minimum. They'll type "lupus", and give up if they don't find what they want. Or "Tom", and stop trying if they don't find Greg and Lupus as well. And then resort to finding the information through whatever roundabout means they last succeeded in.
Beware the average user's depth of knowledge of your system; many don't understand how to use tabbed browsing, and possibly more use Google to find the Facebook login page despite the login link appearing at all times when you're logged out. If you're trying to get as many people as possible, you've gotta plan for the bottom to be as functional as possible. And there are more at the bottom than at the top.
Essentially every laptop / tablet / whatever-PC in use in the medical field works similarly - they use one application which handles all relevant data, and it doesn't search outside that. And this is on a PC, which is essentially wide open to application interoperability (if not as easy to do). Developers and users have already apparently decided that it's not as important / desirable as many would like to believe.
Granted, there are cases where this is useful, but it's a fundamental iOS / Android(/Unix) design choice. Android favors interoperation through APIs, iOS favors using one application for the job at hand.