He's not limiting his vertical process by level, however, because there aren't enough AP to fill the vertical levels for anyone.
The mock up looks like each tree has about the same number of options an entire character class has now, or close to it. The versatility gained is in the ability to mix and match the trees and free PrE abilities. That provides for a lot of options and versatility when a person can choose which trees are in the list compared to not being able to choose which trees are in the list.
No pure class will be able to go very deep in multiple trees either because trying to due so still limits the higher tiers because no one will have the AP to do that. I don't have enough points to develop 4 trees to go beyond the level caps created by multiclassing any more than the multiclass level caps would be at.
Going pure means being restricted to going deep in 1 tree and spending points in the others while having a very restricted choice on what those other trees are. Going triple class means being able to go deep in the race tree and filling that out the other trees with a wide variety of the tree choices and PrE's available. That is versatility there and it's not restricted by depth going with the race tree. It's not even restricted by not being able to go deep in a class tree instead by using a race unlock. The end result is still going to be pure classes can develop one tree and supplement it, multiclasses are going to have the option of developing one main tree supplementing it with a much wider variety of options. Depth in multiple individual trees is a pipe dream.
When we are already looking at a lot of versatility (and I see A LOT of versatility in multiclassing compared to pure classing) asking for more choices is pointless when a person cannot fulfill all of the choices he or she already has except to munchkin up. What you are claiming as a need for versatility I am disagreeing with because the need for versatility is already fulfilled sufficiently with the ability to freely choose which trees will be available.
Bottom line: Someone is not choosing to limit vertical progression by multiclassing in the first place. The vertical progression is still available and heavy vertical progression in multiple trees not a real option in the first place to have given up for anyone. The versatility already exists in the choice of trees and more is not necessary. Having more choices than a person can possibly make use of but still asking for more choice is pointless unless there is a goal in place to improve the build, which is munchkin building.
I expect to have vertical limits on a pure class because I don't expect to have 150-200 AP to fill the existing number of available trees.
