Just curious, not trying to solve any problem I'm experiencing.
Best answer by ratty
I say 'in many circumstances' because there is a situation where the wire is ignored. This is where the STP root path costs for the two units in the pair differ by less than 10. The simplest way to check whether the wire is in use is to look at the Network Matrix at http://x.x.x.x:1400/support/review.
(The wire can be forced into use, in all cases, by disabling the radio on one of the pair. However this is an unsupported configuration.)
Note that in 'Standard Setup' the wire is useless as the Ethernet ports are disabled.