To answer the OP's original question, it is NOT absolutely necessary that you have a passcode. It is simply personal choice, and as you have heard, the recommendation of most users is to have a passcode set.
You may also want to look at the option under Settings > Passcode > Require Passcode, and set this value to something other than "Immediately". For example, when set to "1 hour" the iPad will not prompt you for the passcode again if you have previously used it within that timeframe... You do however "lose" these options once TouchID is enabled.
Remember also, you can train the iPad TouchID sensor to recognise all of your own fingerprints AS WELL as your Wife's. Many people forget this and choose not to use TouchID when the device is shared between family members.
Whatever you decide to do, I hope you end up with passcode settings which suit you and your wife!