yeah, some of it does make sense because of how the steel moulds work, simpler to but not the same as rubbers, but I've seen bits where it's all on a flat front line where.. there is no reason why it's cut like that. Not really plastic, but I've got a resin one which was SOO hard to deal with because they put a hand with a walking stick as a second piece despite the fact that it is on the same flat plane as the rest of the model. there are no undercuts or anything.. in fact, when it's glued in place, it even joins up with the tab which would have made it even easier for casting. There are times when yes, I will say it is better to be in multiparts cause of ease of removing from the mould or for quality (like if you really don't want to put the cut line on a certain plane.