Hierarchical menus help with app discoverability by showing all the apps the system has to offer.

I've always found such menus more confusing than anything.

Can you guess under which menu is the termina/command line program is hidden? No, unless you know in advance that it's under "Accessories". I mean, why isn't it under "system" or "programming"? It's too hard to guess what the menu author(s) were thinking so it's always hard to guess where a certain program is.

Think of the first time you ever used a computer. I agree that once you know that there is a app called terminal, using the Hierarchical menu might not be the best way to launch/use it. But the very first time you use a computer, its useful to see "Ahh - I can do Graphics, Programming, System Administration, Games. Oh wow it even has Solitaire!". So a hybrid approach with something like Gnome-Do for power users while retaining the hierarchical menu for regular users is optimal imo. Heck a lot of power users even create application sessions and do most of their app launching automatically at startup.

You can view all the apps installed just fine. Ubuntu, unlike many distros, doesn't install 10 of every kind of app so I don't think its necessary.

Exploration, with a menu I can explore and see what is there. Search only means I can randomly type stuff in and see what happens but I never know the extents of the system.

You should check out a review if you haven't actually used the UI.

http://maketecheasier.com/ubuntu-natty-beta-1-review-screens...