There is no need to have a 'great spread' though. These six cabinets have probably been chosen carefully by the 'Chief Engineers Office'. I would think six different cabinets worth of test data, is enough to draw conclusive results.
BS, I think the CEngO will already have access to more than enough data to draw conclusive results about Vectoring as a technology. Surely this is more about trialing the practicalities?
A cynic might also observe that Braintree and Barnet do not particularly represent the wider UK. However, I accept you have to start somewhere, and convenience is not unimportant.
Although I perfectly understand and actually sympathise with BT's position as a commercial entitity (not of it's own making), it is understandable that informed 'consumers' will have anxieties about the implications of BT's past (perhaps short-term) commercial decisions. It would serve BT well if it responded positively by explaining the extent to which this trial will or will not address their concerns.
[Edi] For example, of all the potential variables relating to the rolling out of Vectoring in the future, which of those will the trial actually address, and what 'success'criteria have been established beforehand in order to judge the success or otherwise of the trial? Or is all that 'commercially sensitive'?