The man who wasn't there said:
20 year old IBM Model M keyboard I pulled from the garbage bin
Seriously! I wonder how large a fortune could have been accrued if all of the give-away / throw-away Model Ms were sold. As they say, "one man's garbage..."
Overweight Manatee said:
Higher DPI is still useless because we have this thing called a mouse sensitivity setting which does the exact same thing.
No, no you don't (and I say this without having a gamer-chic mouse to defend.)
Mouse "sensitivity" changes how much the mouse reacts to each inch of movement, if you will. An increase in the actual DPI, or sampling rate, of the device lets you take the same user-adjustable "desk to screen" travel ratio and break it into smaller increments.
Take a lower DPI mouse, and turn down your sensitivity to make minor adjustments easier - you now have to cross more physical space to make the cursor react the same amount, but you gain back the precision lost by otherwise assigning too much screen space to too few dots in a smaller mousing area. Crank the sensitivity up, and you can now cross the entire screen (or "do a 180" seems more relevant) in the physical space of an inch, but finer movements are lost. In the extreme - as in, "higher sensitivity than anyone would let a user set" - you'd be skipping pixels per each dot, much like how you skip dots for each pixel at a low sensitivity.
Both have their trade-offs, and are a matter of comfort and preference - hence it being a slider since the dawn of man, and "gaming mouse" manufacturers tacking dials on their mice to adjust it on-the-fly. A high DPI neatly (but not cheaply) side-steps the entire issue by sampling - I swear I'm not trying to be condescending - by sampling more dots per inch. Your, say, 1680x1050 pixels of screen-space (and the sub-pixels between them in vectored or trigonometric applications) can now all have direct mechanical representation in a smaller space.
In another extremized example (but surprisingly a real-world one!) Razor sells a 2000dpi mouse. Theoretically this means I could turn off the OS mouse acceleration, still be able to cross the entire screen in under an inch, AND accurately point to any pixel I wanted within that space. (That's presuming my motor skills were up to that lofty goal, which I doubt anyone's are - 2000 DPI is probably a bit much, but "PC gaming markets gonna PC gaming market.")
Thinking you'll get the same results (regardless of if you find them necessary in effect or not) from a "sensitivity adjustment" on equipment that is, by definition, sensing less reminds me of the non-scandal in the digital camera market. Imagine the consumers' surprise when they learned that stretching the same number of actual optical sensors across more pixels didn't result in a higher-quality image!
Long story short: if the information just plain isn't there, making each piece of it count less is a far cry from having the full story.