jdb supports the dbgtrace command line option for enabling JDI debug tracing. The problem is that often you don't know that you need the tracing until after you've already launched jdb. Another issue is that even if you know you'll need tracing at launch, often you don't want to see any tracing until after you have done a few things in jdb. The trace output before then just becomes a nuisance.

Having a dbgtrace command would allow you to change the trace flags at any point during debugging.