In today's scenarios, having both is always a good idea. Let's forget the flight map or music, but a few considerations...

Any delays... This is especially true in the winter season... you can get stuck at any stage of a trip from check-in.... to security.... or waiting for the flight... to boarding... to preparing for takeoff... to deicing... to flight path change... to waiting for a gate... to immigration clearance... to luggage... Now, hopefully you're paying attention for the important stuff (like security, immigration or luggage), but you likely don't have enough video to last you through most of these (assuming international flights)... you're likely to run out of stuff if you depend solely on your tablet.

As others have mentioned, except for a few airlines who have tablet holders built into the seat in front of you (and most of those will not have an IFE anyway) you have to take up precious tablespace and an unhealthy angle to watch the tablet.

Depending on the length of the flight, tablets require supplimentary power. Not all airlines supply this additional power to all classes so it's possible to run out of power (especially at critical points in a movie)...

Assuming you depend on streaming (no downloading), not all airlines can provide sufficient bandwidth (if they do at all) for the flight. Lots of deadspace out there or in some cases, too many people using the in-flight wifi...

Mood change... sometimes you change your mind and don't want to watch what you've downloaded (or can't stream)...

Just plain discovery... I can't tell you how many times I discovered something new or interesting rummaging around the IFE (either before the flight or during)...