The logging feature in mIRC 6.20 worked in the same way as previous versions, so you must be referring to mIRC 6.21.
In mIRC 6.21 the logging behaviour was changed in order to resolve the corrupt/mixed log file issues that many users reported when using multiple server connections.
mIRC 6.21 resolves this issue by ensuring that log filenames are unique for each status/channel/query/chat window and are only afterwards merged into the main log file.
In previous versions if mIRC or Windows crashed, your whole log could either be lost or corrupted. In the new version, only the non-merged part of your log might be lost or corrupted during a crash. This means that your main log always remains intact and only the most recent log might be lost.
In the event of a crash, you can go to the View Logs dialog in the Tools menu and merge any left-over logs if you wish.
I could make mIRC automatically merge left-over log files in the next version, however if the left-over logs are corrupted that might lead to the same issues as before.
