Good idea in principal, I am sure it will have standard key function to resolve any power or stolen phone issues, and if it can work over wifi then it will help when :
- other occupant loses phone or key open remotely
- tradesman or none occupant requires entry to the house
- verify that the door is indeed locked
- possibly monitor when the door has been opened, when on holiday or away from the house or at night when in bed
as most burglaries are opportunistic, a passing tea leaf spotting an open window or door the problem does exist, this in some way could help with this with a similar device for windows then you could have something quite useful.
All that said price would be the issue