I think you misunderstand how it works. You're still logging everything. Your "current" logs are just put into a .1.log file instead of a .log file. When you part the channel, close mIRC, etc, then the .1.log file is merged with the .log file. You still have everything... you just might have them in different places (old log before parting in .log and new log after rejoining in .1.log). You won't lose anything and you will still have access to all of your old logs.
Hmm? I'm still getting .1.log (and sometimes .2.log and .3.log) files that end up never getting merged... With sessions before and after the main log. That's partly why I tried to script my own logging system that logs upon EXIT. But I've talked enough about this elsewhere