Staggering APs can potentially when help with small area of low coverage could be covered by an AP on an adjacent floor rather than adding another AP on the deficient floor. Perhaps some designs are done to minimize the # of APs, and cross floor coverage is relied on to really minimize the hardware count (not worth it in most cases IMHO).
What if the building floor plan works out just right for AP spacing and you can get a great design and then duplicate it for the rest of the floors (stacking). Is it worth skewing your great layout (hence making a not as great layout) on every second floor just so you can stagger instead of stack? Probably not.
Sometimes the great layout is (for example) a triangle. Well, every floor could be a triangle just inverted every second floor and you get both your great layout and staggering.
Most of my staggering potential sites are hotels. If APs are getting done in the hallway, then sometimes staggering is suitable, sometimes not just depending on how the design works out. If the APs are in the rooms, then staggering is usually easily done without compromise. Personally I'm not willing to create a notable area of poor coverage one one floor just so I can stagger.
