The trees in the MK's hub were designed to push the castle further back; to give the illusion that it was actually taller and farther away when you first saw it from Main Street. The hub itself was also used as a cozy, shady, quiet transitional area. You couldn't just see Tomorrowland via the Adventureland bridge, or vice versa. The hub trees were used as "stage curtains" to help slowly reveal the next land you came upon as you navigated the hub.
@WDW1974 knows all about my fascination with the hub trees. The spirit tried to get me to go to a free clinic to resolve my sorrows about the trees being removed, but we ended up going to a Fuddruckers instead.
Here is a picture of the hub in 1991, granted the trees were overgrown in this photo (they did get trimmed from time to time over the years)
This picture from July 2003, shows the hub trees as well as the large trees before you get to the hub that were removed for the castle projection show.
The hub trees were removed in September 2003 to make way for Wishes in October 2003.
The hub was then redone/flattened out (original 1971 circular faux brick benches, planters, etc were all removed around 2005 for the global 50th anniversary of Disneyland.
And here is the hub today. Barren, flat, with a Castle plopped right down at the end of Main Street saying "LOOK AT ME I'M HERE!" with no sense of foreground (the trees) to push the castle further back and also help draw you toward the castle as you could see it peaking up from behind the trees.
The hub is extremely barren, and doesn't help in of all places...FLORIDA. Hopefully the new hub's outer ring of trees will help provide some shade at the very least.