Why is a turbofan engine less efficient when air leaks over fan blade tips?
If it was to leak, it's still going through the intake to the compressor or bypass duct. 


This is incorrect. A "leak" would cause the compressed air to circle around the fan and exit again from the front, reducing the efficiency of the fan, because part of the power would be used to compress air that does not do any useful work in the engine.

The circling around is a consequence of the work the fan itself does: after the fan the air is at a higher pressure than the air in front of the fan, this means that the air outside is pushed back by the air inside, and if there is a leak, the higher pressure air (inside) leaks out from the front.

Minimizing this "leak" helps maximising the efficiency of the fan, and of the engine as a consequence.