with specialized stems,as camilo said, the stem shim fits on the fork square. you don't need angled spacers.
the shims allow the stem to be adjusted at different angles without having to buy new stems. you only have to buy new shims, or just flip it upside down, to change the angles.
with specialized top caps the only reason they are shaped differently is to put pressure on the stem and the shim. with there angled shims the stem is larger and standard top and won't put pressure on the stem. without there top cap in RARE instances the stem can loosten from the shim alowing the bars to twist on the fork.