why would you want to clear the log, unless you are trying to hide something from the router owner ?
it's simple - long log take long time to load and winbox response slows down terribly! takes up to 10sec to just show the log window - and this on broadband connection!
create new command (PLEASE!) - if somebody wants to hide something - he can just simply restart router. but when i want to start log network events for debugging - i want to do it fast and from fresh log - it's just common sysadmin unixway style - "clear" or "echo > log" - not that bunch of commands it needs now.
you can add proper ACL for log clearing for example to sort out problem with unauthorised log clearing