I suppose you've hit on why things are the way that they are ....
Three phase is pretty unusual in residential areas. It's almost unheard of to a house itself; before the PoCo will supply three phase to a house, there is a tremendous amount of paperwork.
When three phase is an afterthought - that is, the PoCo didn't expect for there to be a need for three phase at the site, and the three phase loads are specific - then the PoCo is likely to save a few dollars, and create it using only two transformers. Again, a large part of the paperwork the PoCo will want will focus on exactly how the delivered power will be distributed.
Unless he can see the transformers up on a pole, there is no way for a sparky to tell if he is dealing with a "delta" or "open delta" service. The only indication that a leg is overloaded will be the voltage drop during peak loads.
For economy, the PoCo likes 'open delta' three phase services. That's one less transformer to buy. By any other criteria, the "Wye" style is much preferred : three transformers, every leg the same voltage to ground.
For the "dryer" example, we look back to the difference between an "appliance" panel and a"power" panel. A "power" panel, in practice, lacks a neutral - so there will be no 120v loads. This is one case where it pays to read all that fine print on the panel labels
