It's certainly interesting that nobody seems to bat an eyelid about not being able to buy a through ticket to South Shields.
Funnily enough, I have been frustrated about not being able to buy a through ticket to Bury (Lancs) in the past. The daysaver alleviates this, but you can only get that from within the boundary.
That's because people are used to it not being possible; Personally, I think it would be an improvement if it was possible to buy through tickets to/from any Tyne and Wear Metro station.
We should be aiming to make public transport more integrated, not less.
Also you don't tend to get the same sorts of "buffers halfway along the line" situations as you get with Merseyrail. It would presumably be quite irritating for someone in Burscough, for example, to have to undertake two transactions just to get to the centre of Liverpool.