Yes Xeon is cheaper, cooler and has generally higher single-thread performance but still there is no reason to give your competition any unnecessary edge.
The x86 core that goes into a 150W high end Xeon is the same core that goes into 5W laptop/tablet chips. Going to 4 threads per core is probably a win for Xeon but how much does it hurt the mobile end of things even if the feature is disabled? The alternative of doing a separate 4 way SMT core just for Xeon throws away a huge NRE cost advantage vs RISCs.