Smartphones seem to have mostly sacrificed some of their security for convenience.
Take Windows for example, sure you can setup multiple user accounts with different levels of privilege , access to website and apps etc but how many people outside of a corporate or academic setting actually use this?
Whenever I borrow someones laptop they just use their own login, sometimes I find porn in their Internet history but at the end of the day who cares?
Perhaps this is more of a problem for people with kids who might want to use the internet themselves but when their child uses it they don't want them to have access to certain sites or see that their parent has accessed certain sites.
One issue I have with android is that when I clear the history in the browser and delete all cookie etc etc.
If I hit the back button it still goes back to whatever I visited last , also if I goto google and tap the search bar all my previous searches come up. It's not really very privacy friendly.
Hopefully this problem will pass once everyone has a smartphone so they don't need to borrow someone elses.