I once met a 747-400 after it's delivery flt from Boeing, it went straight in the hangar and we changed all 4 engines. The reason being, amongst others, that if you have 4 different engines at different operating hours then you didn't have to do the same task on all 4 engines at the same time further down the road, saving unnecessary downtime. At certains in their life they are removed for overhaul and you don't want to be doing all 4 on one a/c at the same time.
I personally hate single engine taxi, the excessive amounts of power required to manouver on one engine, increases wear, increases the amount of fuel used (I don't believe any is saved because of this) and more importantly he hazards to people on the ground caused by increased jet blast, noise etc.
I saw a 757 the other day trying to do a right turn onto a stand with the right engine only, they must of been at take off thrust trying to get it round the turn.
