IIRC, platinum (and iridium, and I'd guess other exotic metals) plugs use a very fine wire for the center electrode, due primarily I'd guess to the rarity and expense of these precious metals.
While many of the physical dimensions of spark plugs haven't changed in the last 45 years, ignition and engine management systems have. Though small diameter, the electrode tips are designed to burn off contaminants and oxidation quickly (just like a MAF sensor, I'm told). And the smaller electrodes perform pretty well with modern vehicles' more precise engine management systems (more consistent/precise engine temp control [H2O], knock sensors, etc.).
Add to that the fact that head and combustion chamber design has evolved to the point where access to spark plugs is so limited on some newer cars it's darn near impossible to R&R. And manufacturers want to increase maintenance intervals by a wide margin (50K-60K miles before first plug change), so market demands have also driven plug design, primarily for longevity IMHO. That's why we see so much platinum, iridium, unobtanium, etc. in electrodes. Under these (more) tightly controlled and precise combustion conditions the smaller electrodes can be used for more miles compared to copper.
But in an air-cooled engine with far less precise engine management (and, um, uniquely shaped piston domes) the larger electrodes offered by copper plugs seem to provide more consistent and reliable ignition -- at least that's the explanation I take from the anecdotal evidence of copper plug success I read about here.
Just my observations, YMMV.
http://forums.pelicanparts.com/porsche-911-technical-forum/732702-new-plugs-couple-questions.html
http://forums.pelicanparts.com/porsche-911-technical-forum/597497-platinum-4-experience.html