Why do I need a staff if I want to cast Cone of Cold or Ray of Enfeeblement? It's just random design. It's arbitrary.
For one thing, I think the designers for 4e have been reading
The Dresden Files.
In
The Dresden Files, wizards can cast spells, sure. But having
foci like a staff or other item allows them to channel magic through it to compensate for what they don't have. The main character has a lot of
raw power, but very little precision - a fire spell without his foci ends up looking like a bomb blast. Bad if you're an ally or a bystander. So he uses a rod to funnel his magic into a single beam, which is much easier to aim than a huge torrent of flame.
Wizards craft their own foci; some use rings and bracelets, others use gloves, chains, daggers, etc.
In the same vein, I anticipate that we'll see a plethora of other Implements, and it isn't beyond the scope to just mix and match. While I understand why cones come from the staff (it's a brute instrument like a shot gun) and long range manipulation comes from the wand (it's the magical equivalent of a surgeon's scalpel), mixing the two can make sense, or switching them out for other items that make more sense to the wizard in question. And so you can have some variation (to avoid the inevitable "Look, he's got an orb - he's a wizard! Kill him!")