Suppose you see a menu where each option has a hot key associated. Do you navigate to your desired option using mouse or arrow keys, or press a hot key? Maybe WIMP interface is more straightforward, but pressing a hot key is faster and easier in the long run. Generally, the more of the keyboard is used (that is, the more keys), the easier is it to perform things for a player who has mastered the power of the keyboard in general and the keyboard layout of the particular game. Leaving a keypress or a keypress combination meaningless is a kind of loss (unless the set of actions in the game is already "saturated" by assigned keys). Roguelike designers often don't implement WIMP interfaces at all, seeing how much more cumbersome they are.
In traditional roguelike interface, I just have to press 2 keys to perform an action like dropping an item. How many keypresses, clicks, and mouse movements are required in the interface you suggest? I have played POWDER which uses something like that, and it was much more cumbersome.
Regarding pick-up versus search: it's not that obvious that when you are standing on a item, you want to pick it up, and otherwise you want to search. What if you want to search in a room filled with junk? IMO it's also more logical to have separate keys for such different actions. Maybe Fatal Labyrinth was designed in such a way that it is obvious in its case, but most roguelikes are not. Thus, they do need a separate key for searching. It would be possible to wire the pickup key so that is performs search when there is nothing to pick up, but why do that if there is a search key anyway? Besides, many traditional roguelikes (e.g. Crawl) have auto-pickup feature, which means that you don't even need to press a key to pick up.
On the other hand, what should happen if the user presses a movement key, and he obviously can't move in the desired location? Doing nothing would be a loss of keyboard power. Thus, doors are opened, and monsters are attacked.
