I know at least one person who does this ... I can't say i agree with it but it's his choice (and his pain if he gets caught either doing this or not having a valid ticket ... especially since they have started penalty fares around here).
Some of my other friends are lucky as they get free rail travel due to their work contracts but I just have an annual season ticket so I don't have to worry about getting a ticket for my commute.
It's impossible to be caught though.
The ticket, providing it hasnt been written on or stamped is valid for a month.
If it hasn't been checked, it's still possible to use it (unless you're an angel).