Phase cancellation is going to occur any time you have 2 or more speakers. But it doesn't stop people from playing with multiple speakers. I always use 2 cabs on my 2 different rigs. I get a tighter low end, a bigger sound with more coverage. On my Pig rig I use a stack, having the speakers vertical so close together makes it so side to side movement reveals no change in the sound, minimizing the cancellation effect. Michell donuts help also to minimize on axis directivity. making my rig the best I've heard for equal sound dispersion to all areas in and around the stage.
On my other rig, I put one cab on each side of the drums, so both sides of the stage can hear me. Bass, singer etc. Now that set up has a zone of cancellation that probably I alone notice, standing right in the middle. But the guys in the band love the sound, and we get a great mix in my band room. I also use the donuts on these cabs as well.
I have not noticed a issue with the ohmage thing. Boogies are designed well, and quite possibly sound punchier at 4 ohms.
These are my personal experiences in "The ext cab zone".