enable-thread-safety defaults?

Is there any actual reason why we are building without thread safety
by default on most platforms? Seems I get asked that every time
somebody forgets to add a "--enable-thread-safety". Wouldn't it be
more logical to have that be the default, and provide
"--disable-thread-safety" if there are platforms that still don't
support it?

AFAIK pretty much all binary packages will do it by default, but it's
easy to forget when building from source....

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/