I think it's a balance though - if you've got a city which can build onagers and is on the "front line", then sure, build some and let them "spill out" and conquer the neighbouring territories, that will definitely save you some time. However, it is much faster to wait 1-2 turns building rams/towers than marching your onagers across a long distance, which might waste 6+ turns.

For example, don't use onagers to get to that city with Amazons in it (the one where you have to walk round a forest) - it takes long enough with infantry!