While I agree the mortars are a pretty undesirable feature of the game right now (on par with arena territories for me in terms of disappointment when discovered), I'll add a few notes from the earlier levels. For context, I switched to beating older levels for the mine bonuses after completing Europe 1066 on first ascension (and while waiting for the 5.14 update to be available on IOS).
* The mortars seem to have 100% accuracy for adjacent territories on early levels and are a bit cheaper than using late-game mercenaries (assuming no discount mercenaries advancement investment) on Final Earth and Peloponnesian War.
* On Final Earth, I ended up using it twice, for decent value, though wasn't in danger of running out of mercs.
* For Drakemor's world, the location seemed pretty bad, so it was the one of the last territories I conquered.
* For K-PX I probably could have saved a few minutes to maybe an hour (max) on playtime by using the mortar since I miscalculated and ran out of mercs near the end.
Since I actually have 20% bonus mercs, my takeaway is that the mortars would likely have a non-trivial savings the first time around on certain early levels. Hopefully they'll be improved with time - the 1 hour load time is by far the most annoying aspect from an enjoyment perspective IMO.