Actually, both multiple and unified have source jars.
Not exactly:
using multiple.target, and it's includeSource tags, PDE searches for
*.source bundles additionally to any bundle. So all relevant
source bundles PDE can find in the site will be part of the target
platform. It's pretty useful at dev-time.
In unified.target, there are some source bundles since we
include some .sdk features (include source explicitly). However, the
PDE mechanism to get also sources for anything doesn't work, since
it relies on our TP site, that does not include all sources for all
bundles.