I think we're splitting hairs a bit with the 2-3 vs higher thing. A good mix is what will make Central PHX a walkable, urban place. I don't think PhxDowntowner is saying everything needs to be giant condo towers like 44 Monroe or Summit*.
A key thing that hasn't been brought up in this is the form of the buildings. Hopefully the Urban Form code will help with this, but we can't have any more St Croix Villas or The Met, suburban style complexes. They're walled off and just look out of place.
My neighborhood here in Boston is very walkable and we have a huge variety of densities:

2 story. This street is all 2 story duplexes. 2 story buildings can be done densely as seen here.

About 2 blocks from me there's even a random 14 story building

My block has many 3 story apartment buildings that are plenty dense

My building is the tallest on our little street at 5 stories. Most on our street are 3 and 4 stories.

This is an aerial of my block, its pretty dense. The only non dense area is the Mosque and its small surface lot. Downtown needs many blocks at least this level of density to succeed.

PHX does have some pockets of good density. These are in Mid and Uptown respectively. Sadly they suffer from things like cul de sacs & don't have enough adjacent walkable retail even though their densities would support it.
I'd love to see the neighborhoods outside the 7's (Lower Grand, Capitol Mall, Garfield, Eastlake) fill their empty lots with complexes that are 2-5 stories. Anything more than that probably doesn't fit in with the neighborhood character. We need more of these in those areas:
The Downtown core should probably stick to 4+ story stuff, with occassional smaller stuff thrown in. Have Downtowns feeder neighborhoods focus more on the 2-4 story developments.
What we can't have is more of these sorts of developments:
They thing is filling up the dirt lots. If 25% or so of the buildings are 2-3 story, thats probably an OK mix.
*which btw, is this going to become apartments now too? Seems like whoever is in control over there would be wise to rent the empty units until the condo market comes back in a few years.