Also possible implications for learning lots of information, cabbies with "the Knowledge" had worse visual information recall.

From BBC:

As would be expected, they were better at memory tasks involving London landmarks than the non-cabbies, but this advantage appeared to come at a price, as the non-cabbies outperformed them in other memory tasks, such as recalling complex visual information.

From the paper:

The memory profile displayed by the now qualified trainees mirrors exactly the pattern displayed in several previous cross-sectional studies of licensed London taxi drivers [3, 4, 20] (and that which normalized in the retired taxi drivers [21]). In those studies also, the taxi drivers displayed more knowledge of the spatial relationships between landmarks in London, unsurprisingly, given their increased exposure to the city compared to control participants. By contrast, this enhanced spatial representation of the city was accompanied by poorer performance on a complex figure test, a visuospatial task designed to assess the free recall of visual material after 30 min. Our findings therefore not only replicate those of previous cross-sectional studies but extend them by showing the change in memory profile within the same participants. That the only major difference between T1 and T2 was acquiring ‘‘the Knowledge’’ strongly suggests that this is what induced the memory change.

Ah, I was wondering what the penalty was for The Knowledge. Brain-training is usually zero-sum.