There's really two reasons SILICA is licensed by the wireless card. One is to keep honest people honest, and the other is because we find that the particular card we ship to you, our amazing SILICA customer, is absolutely the best for the job.
When picking the Ubiquity SR-71 card we looked for something that had external antenna jacks (we're not going to solder your cards) but which could also handle the multitude of wireless modes SILICA uses for extended periods of time.
As Mark put it in a recent customer support email:
The Ubiquity SR-71 is the only card we can
support because it's actually the only card that passed all our
tests in search for the perfect wifi adapter for SILICA. All other
cards (including the Alfa) have stability issues when put into the
different modes that SILICA relies on (monitor mode + injection, AP
mode mode with multiple virtual interfaces, AP mode with injection
and an interface in monitor mode, etc).
On the bright side I can tell you that the SR-71 won't let you
down. After all the tests we ran on all the other cards I
personally only use the SR-71 for everything I do. Even when using
the standard antennas that ship with it the card has an impressive
range and you can always swap those out for another antenna plus
amplifier if needed.
As a side note, we're really excited about the next release, which will include STALKER support!