What would be the benefits of deploying a hybrid enterprise telephone
system (tdm and voip) vs a native VoIP solution?

It comes down to HOW relable do you what the phone system to be?

VoIP will fail if:
->Network Connection Fails
->Power Fails
->VoIP server fails

Most people are use to if the power goings out STILL being able to
pick up the phone and have it work (call their kids etc...) POTs
(Plain old Telephone System) gives them that but it's more expensive.