You buy the phone and nowhere does it say they promise to update it for you, When was the last time you got an update on a Nokia 5110?
Yeah but we are in 2012 and phones are running much heavier OS and have internet connections so security and the security updates are far more important now. We have banks now rolling out apps to use on phones to do banking.
With a Nokia 5110 could you identify a defect in the OS? With my phone I can clearly say that it has CVE-2011-3874 and Google does have a fix for it https://code.google.com/p/android/issues/detail?id=21681. so some researchers have found the issue and done a CVE and Google has acknowledge the defect and created a patch for it.