One very important thing to keep in mind is that subpanels help reduce the length of branch circuits and thus reduce voltage drop. If you are installing the subpanel right next to the main panel in a new house then you are doing yourself and the customer a disservice. In new construction, subpanels should be placed on a different level and/or a different end of the home (relative to the main). Make sure one of the panels ends up close to the more heavily loaded areas like the kitchen. In an existing/fully-finished home, placing the sub next to the main may be your only reasonable choice.
I don't believe voltage drop is a real issue in many homes but a competent electrician still must take the issue into account. Having 200' long branch circuits in a home is something to be avoided!
Can you name even one case when you are going to use 50-60 circuit spaces feeding branch circuits in close proximity to the panel? If the loads are farther away then a subpanel is in order anyway.