Both have their points. Wood will swell or shrink with the ambient moisture. And every time you drive a nail or tack, you damage the last...imperceptibly, but it adds up. Lasts that are in constant usage, especially where pegging is used (and didn't I see a Bestetti with pegged waists?) will become nearly useless in time. The whole waist area will be "gouged" out. On the other hand, build-ups and modifications are much more secure on wood and more easily blended into the lines of the last. Plastic is self healing. It is stable. But even with the best contact cements, build-ups, especially those along the featherline, seldom adhere to the last properly for longer than one use.