The problem is that it's an irrelevant requirement. Forcing people to post "I concur" doesn't tell you
anything about their level of participation or "activity".
The value that I've always seen in that clause is that it establishes a clear, written minimum expectation for the Justices and makes it a bit more difficult for Senators to waffle on clearly justified votes in favor of impeachment for fear of upsetting someone. Of course, this was entirely ineffective during Torie's first impeachment hearing...
Yes, but there could clearly be a case where a very active and engaged judge overlooks posting "I concur!" twice and are thus automatically forced to go through a tedious impeachment process that just wastes everyone's time.