Depends whether you believe in preventative maintenance - it might pay off in the long run, but it isn't cheap.
There is not really a lot in it in terms of work, between taking out the engine or gearbox. In a Defender, you can also stick an engine crane in through the door, take the trans tunnel out and sling the gearbox that way. If the engine was a TDV6 in a Discovery 3, you wouldn't even think of taking it out!