Sure, he's cute, but the flaws in his design are obvious the first time he approaches anything but the shallowest of stairs. Also: He has jets, a periscope, a taser and oil canisters to make enforcer droids fall about in slapsticky fashion -- and no voice synthesizer. Imagine that design conversation: "Yes, we can afford slapstick oil and tasers, but we'll never get a 30-cent voice chip past accounting. That's just madness."
And over at Slashdot, some other folks respond. A sample comment that I liked:
That complaint [that Stormtroopers are crappy marksmen] always bugged me. I know of 4 occasions in Episode IV (I won't get into the other episodes) where the stormtroopers fire at the heroes.
1. 3P0/R2 wander across the field of fire - Troopers shoot past them to get the actual targets (exactly as you would expect from trained marksmen)
2. Falcon takes off - Troopers shoot at AND HIT a vehicle moving at high speed away from them
3. Trash Compactor - Troopers aren't able to hit a group of people taking cover a good distance off down a dark cooridor. Concealment and Camouflage but they're able to get pretty close in the few seconds they have before they escape.
4. Deathstar Escape Scene - Troopers miss every shot at the group of rebels who are going to lead them to the rebellion base...hmm, could it be they were ordered to miss?
Every other time we see the stormtroopers fire they hit their targets perfectly.
Obvously, Scalzi was just kicking over an anthill to see the ants scurry. But I'm an ant, so I'll scurry a bit as well! First of all, it takes a sharp eye to catch it, but R2-D2 is not stopped dead in his tracks by stairs; he is quite able to negotiate them, albeit somewhat awkwardly. But this makes clear the failing in John's "argument" (to the extent that he has one; like I said, he's just kicking over anthills here).
Simply put, there is a difference between bad design and something being not good at doing something it's not designed to do. R2-D2 is not designed to trundle all over the place and climb stairs and all that. He's designed to roll around, or be locked into place on a ship, and fix stuff. He wasn't built with the notion in mind of bearing the stolen Death Star plans across the desert wastes of Tatooine; he wasn't built with the notion in mind of picking his way across the bottom of a swamp on Dagobah; he wasn't built with the notion in mind of, well, going up and down stairs. John's complaint here sounds to me like if I were to complain that my Swiss Army knife is badly designed because it doesn't drill holes in wood very well. (And no, R2 does not use his jets to climb stairs. He uses his legs. Watch at about the :55 mark on this video. Sure, he's slow at it, but it's not what he was built for.)
Likewise, if R2-D2 isn't designed to talk to people, why would he have a voice synthesizer? This is like complaining about The Wife's sewing machine's onboard computer not being able to play DVDs.
One of the better comments on the difference between design and utility is in the movie Apollo 13. After the accident on the ship and the determination that there will be no moon landing and now the mission is recovery of the astronauts, a bunch of NASA Mission Control guys are in a room discussing options, and one man from the company that actually built the Lunar Excursion Module (LEM) objects that the LEM is not designed to be a lifeboat; it's designed solely for landing on the moon. And Gene Kranz (Ed Harris) snaps back, "I don't care what anything was designed to do. I care about what it can do."
(But just to geek out for a quick moment: the notion that Star Wars abounds with bottomless chasms unprotected by guard rails is really false, when there's only one such locale in the movies: the power terminal that Obi Wan deactivates in A New Hope. Two things, though: that terminal is probably meant to be serviced by droids instead of humans, and droids are undoubtedly seen as expendable should they fall (and service droids probably have better balance anyway). And anyway, the Death Star later turns out to have been designed by a race of beings who have wings and can fly! Heh. OK, I'm done.)