Older plugins used the "archives" folder for writing, as this was always world-writable. Then in 0.8 we added the "templates_c" directory, which is now our "temporary" place.
Please let plugins always use this directory, do NOT create your own directories. You never know how people configure the basedir, and creating a subdir may fail.
Templates_c will be kept in every next release, we will ensure that.
It would be good style if plugins remove their temporary files, yes.
Thanks a lot for those good questions. Most of the core plugins could/should be reworked to follow that style.
Regards,
Garvin