The Tube is your own personal hell (or heaven) and isn't there to be interrupted.
Unless you want it to be. Maybe you want a chat. Except nobody else wants to hear you chat what can only be complete rubbishy small talk. "Oh i see you have a badge, so do I, do you live in London? Me neither, on your way to work (in the morning) or home (in the evening), what do you do in work".
You're now interfering with my peace and quiet and as the Tube is fairly noisy, you're probably compensating by speaking loudly so now everyone in the carriage can hear you. This just isn't a great place to have a chat.
A "fancy a coffee" badge sponsored by Starbucks would be a better idea, a "Trafalgar Square at 4 o'clock chat" badge could be performance art, a "20% OAP discount at Sam's fish and chip shop between 12 and 2" could well drive custom if the old dears could read all that on a small badge.
To me "Tube chat" is like "Motorway chat".. the impracticality of it should have nixed this from the outset altho it has made for good reading of aggrieved travellers.