If R2's job is repairing and C3PO is mainly for translating, why does R2 have the more circular body with wheels which is probably more inefficient for grabbing tools and repairing? Wouldn't it be better for R2 to have limbs and fingers with a more humanoid-based design? All C3PO has to do is translate what R2 says he could have had the wheels.
C3PO is a Protocol Droid
According to Wookieepedia, their purpose is:
to assist sentients in their relations with one another. Protocol droids were programmed mostly for etiquette, and were often used as translators between sentients or between sentients and computers. They were generally designed to look like the sentients they would be serving; for example, 3PO-series protocol droids were humanoid.
They possessed knowledge about various cultures and customs, both to advise those for whom they were working, and to ensure their own behavior was diplomatic and culturally sensitive.
Hence, because C3PO was a 3PO-series protocol droid, it makes sense for C3PO to resemble a humanoid, as that's who their functioning with. What would feel more natural to you: conversing to someone using a droid that looks like an astromech droid, bending down to talk to it, or talking with a humanoid-designed robot.
R2-D2 is an Astromech Droid
Now, they're described by Wookieepedia as being:
a type of droid that served as an automated mechanic, performing a variety of repair duties and often serving as an adjunct or substitute for a nav computer on smaller starships. Astromech droids could also use the mainframes of larger ships to their advantage. Many starfighters relied on astromech copilots.
Considering as part of their duties they need to go outside a starship, being portable is preferable to get the job done quickly; it would be far more difficult to walk outside on a spaceship than to simply drive around with wheels.
The other part of that quote I would draw your attention to is the fact that they are used on starfighters; these are agile, relatively small craft designed for fighting. If you had a humanoid robot, that requires far more space to fit an astromech droid in rather than the small, compact design they already have.
The OP also makes the point that the non-humanoid design
is probably more inefficient for grabbing tools and repairing
I don't ever recall seeing a point when R2 was required to 'pick-up' a tool. Wookieepedia lists his on-board equipment as including:
- Buzz Saw
- Electric Pike
- Fusion welder
- Scomp Link
- Power recharge coupler
- Booster rockets
So, he's pretty well-equipped without need for any other tools to pick up!
So, no, R2 shouldn't have a humanoid design and 3PO's humanoid design is appropriate when you take into consideration their respective purposes and where they serve their respective duties!