The question (and the following answer) is in their FAQ (
http://sourceforge.net/apps/trac/lightspark/wiki/FAQ ):
"Lightspark started as an experiment in achieving high performance by
exploiting the features offered by modern hardware. Such target is nearly
impossible to reach working inside an existing and large project such as
Gnash. As anyway Gnash performs ok on older clips (those not using
AVM2/ActionScript 3) currently lightspark is able to fallback on Gnash to
handle them.
Lightspark tries to do everything in the right way by following the
specifications released by Adobe. I also think that Flash is a technology
that is here to stay for a long time, both for market and technological
reasons, so a well made (read: not an hack) open source implementation is a
investment for the open source community."
