It gives you a Rating 1 noise reduction, one could easily argue that you cannot stack them for two reasons.
1: The Data passes through the Datajack, not multiple but just the one.
2: It gives you Rating 1 noise reduction, not +1 noise reduction. So with 2 active at the same time, you still only have Rating 1 noise reduction.
As for the abusive situation: With 10k nuyen and 1 essence someone could otherwise be pretty much completely immune to any kind of noise.