The Centro is not redundant: it turns the B4 from a clay munching monster into one that shreds carpet to bits
The B5M was probably based around the C4.2's geometry, weight balance and to a degree also it's flex characteristics.
If you then start comparing the C4.2 vs. B5M for purchase, drivers that already have a B4 must be tempted to go for a C4.2: not only is the conversion much cheaper than a B5M, but they can't use any of their spares on the B5 because that's metric.
And for people running on a tight budget (regardless of whether they have a B4 yet or not), the value for money is amazing: Cheap donor car (second hand, that is) and cheap and abundant spares for a car that will perform just as well as a B5M.