These things will become a _permanent_ part of the nose gear assembly.
There isn't enough room on the main gear; they are full of brakes.
Some airlines have experimented with using the thrust reversers to pushback from the gates. Bad idea; you blow lots of stuff up that then gets sucked into the engines. 'Foreign object ingestion' is a big baddie on jet engines. Using the nose gear for pushback offers another safety benefit: when a transport aircraft is moving in reverse, you really _don't_ want to use the main brakes to stop it; the CG is way too close to the main gear, and there is a significant risk of tipping. If you use the nose gear for breaking as well, then you can't tip back.
I don't know the details of bleed air effect on the engines...I just know that the engineers who work with it despise it.
The jet engines are very inefficient for ground operations. The estimate is that using APU power to move the aircraft through nose gear will save upwards of 200lb of fuel per flight.
The increase in APU power is significant but small. The hotel load on a modern transport aircraft is tremendous.
-Jon
