[sorry for the late response]
If the only symptom was the huge log file then it could be argued that: had
debug messages been disabled, there wouldn't have been a problem... :D
That makes sense, but
(a) I disabled everything in kdebugdialog and truncated .xsession-errors to 100kB a few days ago, and now it's 16MB so still some traffic (over half from kio_xxx). That size of log file isn't an issue (for me at least), though most of the entries appear pretty uninteresting (e.g. HTTP requests).
(b) many debug options are active by default, so new users are going to keep coming across nuisance messages (either in terms of .xsession-errors size or output on command-line when running things like kwrite).
Any more thoughts on how much debug output is reasonable? Obviously it makes some sense for errors and warnings to be reported, but a lot of the output is only "trace" type debug output which is often only useful to people debugging or optimising applications so IMO shouldn't be enabled by default (although feel free to disagree if you have a use for such output). Of my current 160000 line log file, 2600 lines are matched by 'egrep -i warn' and another 800 by 'egrep -i error'.
Where does that leave us? Put up with it as is? Rebuild without debugging output enabled? Just disable all entries in kdebugdialog by default (this solves my issues anyway)? Ask KDE devs what their intentions are (maybe there are some KDE devs on this list)?
Cheers, Diggory