Well its quantifiable on the Android/iOS phones all right -- "zero", with a little better than zero if a customer installs a bunch of 3rd party ($$$) packages. The real question is how many users with a 'security' requirement are actually out there in the enterprise space? Does a government user who merely drives a road grader, really require anything fancy and secured?
One interesting 'rumour' I've heard is that the battery life on these devices blows the Android and iPhones away -- again, likely caused by the platform being locked down and very limited in 3rd party code it can run, in addition to highly tuned/optimized hardware. This could sell devices as well, but I haven't a clue about the impact.