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There are a couple of reasons: some systems don't
have networking installed, and running w/o the
subprocess causes the debugger to work slightly
differently: it is possible to debug IDLE itself. It
is surprising that it was possible to run IDLE in
either mode w/o mucking up the code too badly.

But there is one major problem: on Windows, it's
possible to start more than one listener on a port.
I haven't been able to track this down, it's somewhat
erratic.  Given the problems on Windows with stuck
subprocesses, I'm loath to go to multiple instances
of IDLE w/o fixing the multiple listener problem with
its collisions.

I certainly agree with you that it would be very
desireable not to use the -n switch in the "edit with
IDLE" binding.  And I also agree about the efficacy of
the subprocess, that's why we went to all the effort
on IDLEfork.

https://sourceforge.net/tracker/
index.php?func=detail&aid=661363&group_id=9579&atid=309579

Maybe we can get this fixed in 2.6.  It's way too 
risky to attempt it in 2.5 and it's a new feature.

Could you add your patch so I can look at it?