If the debate takes the track of "two transponders"...can we steer it back to the point of view that ALL aircraft could benefit from an answer...not just airliners.
Remember, there are more PA28's on the UK register than ANY other aircraft type...and I doub't any one of them has a spare transponder...nor a "non handling pilot" to "twiddle the numbers" whilst the handling pilot scans the TCAS.
Please, don't forget your roots when continuing this discussion.