One use for reference-count biases is where the reference count is modified often enough that it must be represented as a per-CPU variable, as can happen when the reference count represents the number of outstanding I/Os. However, unless the reference count tends to be much larger than the number of CPUs, you get no benefit from the per-CPU nature of the variable. Adding in a large bias, on the other hand, allows the counter to operate on a completely per-CPU basis under normal operating conditions.
This trick is useful in cases where it is only necessary to actually test the counter for zero-or-not in special situations, such as when attempting to remove or reconfigure the I/O device. When such a situation arises, one subtracts out the bias, and then runs inefficiently until the I/Os drain and the reference count goes to zero, at which point the device may be safely removed or reconfigured. The bias may be added back in to return to normal operating conditions.
A very specialized use of a reference count, perhaps, but one that has actually appeared in real code in past lives.