Why Use EU to Power Quarry?

Based on the numbers yes the magmatic engines would be more efficient in terms of lava used per amount of digging done. However you wouldn't quite get the full 4MJ/t/engine at your quarry because you'll need some sort of power distribution system (e.g. redstone energy conduits) to connect up all the magmatic engines you'd be using, but the loss would be very minor.

The downside of fully powering a quarry with 100MJ/t is that it will run ridiculously fast and your sorting system will need to have been designed (or hurriedly retrofitted) to handle the huge influx of mined materials without clogging up.