It’s your personal choice on how secure you want Siri your iPhone or iPad to be.
Some people like to use a passcode to prevent access to Siri on their device, others don’t as they prefer the convenience of being able to quickly access their email and apps without any extra steps being involved.
Surprisingly, in its default state Siri will override your passcode.
This means that a complete stranger could pick up your device - or use the 'Hey Siri' command - and make phone calls, send texts, send e-mails, and access other personal information without having to enter the security code first.
However you can prevent Siri from overriding the passcode:
Whether you decide to add this layer of security to your device is up to you.
There have been reports of people being able to display information in stranger's iPhones. Thankfully most of it has been for good reasons - such as returning a lost handset, and over time Siri is getting more intelligent at recognising your voice and not reacting to strange voices.